[mount] Add test exercising refactored btrfs code

src/modules/mount/tests/3.global

@@ -0,0 +1,8 @@
+# SPDX-FileCopyrightText: no
+# SPDX-License-Identifier: CC0-1.0
+partitions:
+    - device: "/dev/sdb1"
+      mountPoint: "/"
+      fs: "btrfs"
+partitionChoices:
+    swap: file

src/modules/mount/tests/3.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.