Since mutter was changed to be D-Bus activatable, the test cases has not
been working when running from inside a GNOME Wayland session. This
commit makes the test work again by ensuring the tests run in a nested
mutter instance.
https://bugzilla.gnome.org/show_bug.cgi?id=763125
Separate from meta-test-runner which runs metatests testing window
manager operations, a new test program (mutter-unit-tests) is
introduced. This is meant to run unit test like tests on various units
in mutter.
An initial test testing the order of MetaLater callback invokation was
added.
https://bugzilla.gnome.org/show_bug.cgi?id=755605