ci: Generate a cover report for the test suite

Based on @ebassi's work from glib!769

This MR allows us to see the succeeded/failed tests each time the CI
pipeline is run.

!528
3 jobs for !528 with wip/nielsdg/junit-report in 5 minutes and 39 seconds (queued for 1 second)
detached
Status Job ID Name Coverage
  Review
passed #266788
check-commit-log

00:00:38

 
  Build
passed #266789
build-mutter

00:01:35

 
  Test
passed #266790
test-mutter

00:03:25