tests: Run pygtkcompat tests separately

pygtkcompat changes the global namespace, in particular patching Gdk and Gtk
overrides. Run the tests in a separate process so that the main tests are not
potentially affected by the pygtkcompat test.

runtests.py runs all tests/test_*.py by default, so rename to
compat_test_pygtk.py and run that explicitly after successfully running the
main tests.

See https://bugzilla.gnome.org/show_bug.cgi?id=688219