Add more test coverage for girepository

I thought I'd help out with test coverage by porting over some tests from the old gobject-introspection tree. These tests come from https://gitlab.gnome.org/GNOME/gobject-introspection/-/tree/main/tests/repository.

Several of those tests relied on introspecting the GIMarshallingTests library so I've changed them as much as possible to operate on entities in GLib, GObject, or Gio, and omitted the ones that couldn't. I've also omitted things that the existing tests already covered.

The organization of these tests was a bit haphazard. I've arbitrarily moved gitestrepo.c and gitypelibtest.c into repository.c; and giteststructinfo.c and gitestthrows.c into their own files. I'm not attached to that division so please tell me if you'd like to see it organized differently.

Merge request includes a few other fix/cleanup commits in the tests.