st/test-theme: Use stage from mutter

Clutter application style stages not supported anymore.

GNOME/gnome-shell!1359
9 jobs for !1359 with wip/clutter-cleanup-and-remove-eglnative in 13 minutes and 13 seconds (queued for 1 second)
detached
Status Job ID Name Coverage
  Review
passed #804966
check_commit_log

00:00:57

passed #804968
eslint

00:01:32

passed #804967
js_check

00:00:57

passed #804970
no_template_check

00:01:00

passed #804969
potfile_check

00:01:06

 
  Build
passed #804971
build

00:10:51

passed #804972
flatpak
flatpak

00:02:09

 
  Test
passed #804973
test

00:00:48

passed #804974
test-pot

00:00:46