Installed tests on Meson
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.