diff --git a/src/calamares/testmain.cpp b/src/calamares/testmain.cpp
index e038b5a2f..6dae0af68 100644
--- a/src/calamares/testmain.cpp
+++ b/src/calamares/testmain.cpp
@@ -18,6 +18,7 @@
 #include "GlobalStorage.h"
 #include "Job.h"
 #include "JobQueue.h"
+#include "PythonJob.h"
 #include "Settings.h"
 #include "ViewManager.h"
 #include "modulesystem/Module.h"
@@ -321,7 +322,7 @@ load_module( const ModuleConfig& moduleConfig )
     QString configFile( moduleConfig.configFile().isEmpty() ? moduleDirectory + '/' + name + ".conf"
                                                             : moduleConfig.configFile() );
 
-    cDebug() << "Module" << moduleName << "job-configuration:" << configFile;
+    cDebug() << Logger::SubEntry << "Module" << moduleName << "job-configuration:" << configFile;
 
     Calamares::Module* module = Calamares::moduleFromDescriptor(
         Calamares::ModuleSystem::Descriptor::fromDescriptorData( descriptor ), name, configFile, moduleDirectory );
@@ -365,6 +366,27 @@ createApplication( int& argc, char* argv[] )
     return new QCoreApplication( argc, argv );
 }
 
+static const char pythonPreScript[] = R"(
+# This is Python code executed by Python modules *before* the
+# script file (e.g. main.py) is executed. Beware " before )
+# because it's a C++ raw-string.
+_calamares_subprocess = __import__("subprocess", globals(), locals(), [], 0)
+import sys
+import libcalamares
+class fake_subprocess(object):
+    @staticmethod
+    def call(*args, **kwargs):
+        libcalamares.utils.debug("subprocess.call(%r,%r) X run normally" % (args, kwargs))
+        return 0
+    @staticmethod
+    def check_call(*args, **kwargs):
+        libcalamares.utils.debug("subprocess.check_call(%r,%r) X subverted to call" % (args, kwargs))
+        return 0
+sys.modules["subprocess"] = fake_subprocess
+libcalamares.utils.debug('pre-script for testing purposes injected')
+
+)";
+
 int
 main( int argc, char* argv[] )
 {
@@ -397,6 +419,7 @@ main( int argc, char* argv[] )
 #ifdef WITH_QML
     CalamaresUtils::initQmlModulesDir();  // don't care if failed
 #endif
+    Calamares::PythonJob::setInjectedPreScript(pythonPreScript);
 
     cDebug() << "Calamares module-loader testing" << module.moduleName();
     Calamares::Module* m = load_module( module );
@@ -406,7 +429,7 @@ main( int argc, char* argv[] )
         return 1;
     }
 
-    cDebug() << " .. got" << m->name() << m->typeString() << m->interfaceString();
+    cDebug() << Logger::SubEntry << " .. got" << m->name() << m->typeString() << m->interfaceString();
     if ( m->type() == Calamares::Module::Type::View )
     {
         // If we forgot the --ui, any ViewModule will core dump as it
@@ -455,7 +478,6 @@ main( int argc, char* argv[] )
     cDebug() << "Module metadata" << TR( "name", m->name() ) << TR( "type", m->typeString() )
              << TR( "interface", m->interfaceString() );
 
-    cDebug() << "Job outputs:";
     Calamares::JobList jobList = m->jobs();
     unsigned int failure_count = 0;
     unsigned int count = 1;
diff --git a/src/libcalamares/PythonJob.cpp b/src/libcalamares/PythonJob.cpp
index 98f284ecc..1a4683c29 100644
--- a/src/libcalamares/PythonJob.cpp
+++ b/src/libcalamares/PythonJob.cpp
@@ -19,6 +19,8 @@
 
 #include <QDir>
 
+static const char* s_preScript = nullptr;
+
 namespace bp = boost::python;
 
 BOOST_PYTHON_FUNCTION_OVERLOADS( mount_overloads, CalamaresPython::mount, 2, 4 );
@@ -242,6 +244,11 @@ PythonJob::exec()
         calamaresNamespace[ "globalstorage" ]
             = CalamaresPython::GlobalStoragePythonWrapper( JobQueue::instance()->globalStorage() );
 
+        if ( s_preScript )
+        {
+            bp::exec( s_preScript, scriptNamespace, scriptNamespace );
+        }
+
         cDebug() << "Job file" << scriptFI.absoluteFilePath();
         bp::object execResult
             = bp::exec_file( scriptFI.absoluteFilePath().toLocal8Bit().data(), scriptNamespace, scriptNamespace );
@@ -319,4 +326,11 @@ PythonJob::emitProgress( qreal progressValue )
     emit progress( progressValue );
 }
 
+void
+PythonJob::setInjectedPreScript( const char* preScript )
+{
+    s_preScript = preScript;
+    cDebug() << "Python pre-script set to" << Logger::Pointer( preScript );
+}
+
 }  // namespace Calamares
diff --git a/src/libcalamares/PythonJob.h b/src/libcalamares/PythonJob.h
index 04a0645ea..af77d741d 100644
--- a/src/libcalamares/PythonJob.h
+++ b/src/libcalamares/PythonJob.h
@@ -41,6 +41,19 @@ public:
     QString prettyStatusMessage() const override;
     JobResult exec() override;
 
+    /** @brief Sets the pre-run Python code for all PythonJobs
+     *
+     * A PythonJob runs the code from the scriptFile parameter to
+     * the constructor; the pre-run code is **also** run, before
+     * even the scriptFile code. Use this in testing mode
+     * to modify Python internals.
+     *
+     * No ownership of @p script is taken: pass in a pointer to
+     * a character literal or something that lives longer than the
+     * job. Pass in @c nullptr to switch off pre-run code.
+     */
+    static void setInjectedPreScript( const char* script );
+
 private:
     struct Private;
 
diff --git a/src/modules/mount/main.py b/src/modules/mount/main.py
index 07166dd75..5e5233935 100644
--- a/src/modules/mount/main.py
+++ b/src/modules/mount/main.py
@@ -74,7 +74,14 @@ def mount_partition(root_mount_point, partition, partitions):
     # Special handling for btrfs subvolumes. Create the subvolumes listed in mount.conf
     if fstype == "btrfs" and partition["mountPoint"] == '/':
         # Root has been mounted to btrfs volume -> create subvolumes from configuration
-        btrfs_subvolumes = libcalamares.job.configuration.get("btrfsSubvolumes") or []
+        btrfs_subvolumes = libcalamares.job.configuration.get("btrfsSubvolumes", None)
+        # Warn if there's no configuration at all, and empty configurations are
+        # replaced by a simple root-only layout.
+        if btrfs_subvolumes is None:
+            libcalamares.utils.warning("No configuration for btrfsSubvolumes")
+        if not btrfs_subvolumes:
+            btrfs_subvolumes = [ dict(mountPoint="/", subvolume="/@") ]
+
         subvolume_mountpoints = [d['mountPoint'] for d in btrfs_subvolumes]
         # Check if listed mountpoints besides / are already present and don't create subvolume for those
         for p in partitions:
diff --git a/src/modules/mount/test.yaml b/src/modules/mount/tests/1.global
similarity index 100%
rename from src/modules/mount/test.yaml
rename to src/modules/mount/tests/1.global
diff --git a/src/modules/mount/tests/1.job b/src/modules/mount/tests/1.job
new file mode 100644
index 000000000..94b3a1492
--- /dev/null
+++ b/src/modules/mount/tests/1.job
@@ -0,0 +1,6 @@
+# SPDX-FileCopyrightText: no
+# SPDX-License-Identifier: CC0-1.0
+bogus: true
+
+# No configuration needed because the partitions are
+# all filesystems that require no special handling.
diff --git a/src/modules/mount/tests/2.global b/src/modules/mount/tests/2.global
new file mode 100644
index 000000000..20aba89ec
--- /dev/null
+++ b/src/modules/mount/tests/2.global
@@ -0,0 +1,8 @@
+# SPDX-FileCopyrightText: no
+# SPDX-License-Identifier: CC0-1.0
+partitions:
+    - device: "/dev/sdb1"
+      mountPoint: "/"
+      fs: "btrfs"
+
+# Expect a complaint and a default btrfs layout
diff --git a/src/modules/mount/tests/2.job b/src/modules/mount/tests/2.job
new file mode 100644
index 000000000..40d8c6171
--- /dev/null
+++ b/src/modules/mount/tests/2.job
@@ -0,0 +1,5 @@
+# SPDX-FileCopyrightText: no
+# SPDX-License-Identifier: CC0-1.0
+
+# Expect a complaint and a default btrfs layout because the
+# partitions use btrfs