Build and test PyGobject with gobject-introspection-tests

Remove our *extra.[ch] test files, since they're now part of gi-tests.

For now it's using a branch on gobject-introspection-tests. Should change to main as soon as the MR is merged.

Fixes #643 (closed).

NB. Should only be merged after gobject-introspection-tests!4 (merged) is merged.