Running each stacking test as a separate installed-test is analogous to
what was done for build-time tests in c6d1cf4a (!442) and should make it
easier to track regressions, by being able to see whether a regression
is specific to one .metatest script or applies to more than one.
Signed-off-by: Simon McVittie <smcv@debian.org>
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2773>
