tests/stage-views: Test whether stage views and frame clocks are freed

Add a test case to check whether the stage views and frame clocks are
all correctly freed after working with timelines a bit and then
hotplugging and removing all monitors.

Part-of: <GNOME/mutter!1632>
13 jobs for fix-frame-clock-leak
Status Job ID Name Coverage
  Review
manual #1080540
manual
check-commit-log
manual #1080541
manual
check-merge-request
 
  Prepare
created #1080543
aarch64
build-fedora-container@aarch64
created #1080542
build-fedora-container@x86_64
 
  Build
created #1080545
aarch64
build-mutter@aarch64
created #1080544
build-mutter@x86_64
created #1080547
build-without-native-backend-and-wayland@x86_64
created #1080546
build-without-opengl-and-glx@x86_64
 
  Test
created #1080550
can-build-gnome-shell@x86_64
created #1080549
aarch64
test-mutter@aarch64
created #1080548
test-mutter@x86_64
 
  Coverage
created #1080552
aarch64
test-mutter-coverage@aarch64
created #1080551
test-mutter-coverage@x86_64