This introduces a MetaContext implementation aimed to be used for test
cases, with as little boiler plate as possible needed in the test.
It currently doesn't do anything, just fills out the GObject boiler
plate and sets a name.
Build it into every core test, for compilation, even though it isn't
used anywhere yet.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1861>
