ci: Run unit and integration tests separately
This seems to make the test runs robust against GDK failing to pick up the Wayland socket on some of the gitlab ci runners. I yet don't know why it only happens there.
There's some cleanups in the tests that make debugging his a bit eaier and remove some old cruft:
* ci: Run unit and integration tests separately
This seems to make the test runs robust against GDK failing to pick up
the Wayland socket (although the root cause why GDK fails to init the
display although the socket is there and functional is still unknown).
* testlib: Check for Wayland socket
Check if the wayland socket is available. Gives a clearer error than
initializing GDK and hoping for the best.
* tests: Drop unused display argument of the shell fixture
We run headless since some time
* tests/notify-manager: Drop unused variable
* tests/screenshot-manager: Drop unused variable
* tests/monitor-manager: Test more API
Closes: #1007 (closed)