[displaymanager] Add tests that run parts of the DM code

- load and set autologin for greetd (this was used to shake out
  code bugs in load/save)
- load and set autologin for sddm

src/modules/displaymanager/tests/CMakeTests.txt

@@ -0,0 +1,10 @@
+# We have tests to load (some) of the DMs specifically, to test their
+# configuration code. Those tests conventionally live in Python
+# files here in the tests/ directory. Add them.
+foreach(_dmname greetd sddm)
+    add_test(
+        NAME configure-displaymanager-${_dmname}
+        COMMAND env PYTHONPATH=.: python3 ${CMAKE_CURRENT_LIST_DIR}/test-dm-${_dmname}.py
+        WORKING_DIRECTORY ${CMAKE_BINARY_DIR}
+    )
+endforeach()

src/modules/displaymanager/tests/test-dm-greetd.py

@@ -0,0 +1,18 @@
+# Calamares Boilerplate
+import libcalamares
+libcalamares.globalstorage = libcalamares.GlobalStorage(None)
+libcalamares.globalstorage.insert("testing", True)
+# Module prep-work
+import os
+os.makedirs("/tmp/etc/greetd/", exist_ok=True)
+try:
+    os.remove("/tmp/etc/greetd/config.toml")
+except FileNotFoundError as e:
+    pass
+# Module test
+from src.modules.displaymanager import main
+d = main.DMgreetd("/tmp")
+d.set_autologin("d", True, "kde")
+# .. and again (this time checks load/save)
+d.set_autologin("d", True, "kde")
+d.set_autologin("d", True, "kde")

src/modules/displaymanager/tests/test-dm-sddm.py

@@ -0,0 +1,12 @@
+# Calamares Boilerplate
+import libcalamares
+libcalamares.globalstorage = libcalamares.GlobalStorage(None)
+libcalamares.globalstorage.insert("testing", True)
+# Module prep-work
+# Module test
+from src.modules.displaymanager import main
+d = main.DMsddm("/tmp")
+d.set_autologin("d", True, "kde")
+# .. and again (this time checks load/save)
+d.set_autologin("d", True, "kde")
+d.set_autologin("d", True, "kde")