This is the last remaining feature necessary to achieve
parity with the Autotools build.

A few changes were made to the install locations of the
tests, in order to better acomodate them in Meson:

 * Tests are now installed under a versioned folder (e.g.
   /usr/share/installed-tests/mutter-4)

 * The mutter-cogl.test file is now generated from an .in
   file, instead of a series of $(echo)s from within Makefile.

Notice that those tests need very controlled environments
to run correctly. Mutter installed tests, for example, will
failed when running under a regular session due to D-Bus
failing to acquire the ScreenCast and/or RemoteScreen names.