tests: Add tests to test gnome-desktop-testing itself
This is the tool for testing most of GNOME, but it's not tested itself, so add a CI job and some basic tests.
We are creating .test files on demand or static files in the source code can be used too, and gnome-desktop-testing is used both to run them and to test itself.
Added also installed tests, all this is meant to be a prerequisite of !12.
Ideally we can simplify something by adding ExpectFail=true
test options, but that's for another MR, here I wanted just to test the status quo.