tests: Add unit tests framework runner

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