Properly reference class types in tests

[Ferada]

C.f. #18 - the tests can fail if the classes haven't been initialised - the proper way might be to have a fixture that calls g-type-class-ref and -unref around the tests which use e.g. GtkButton without creating an instance before looking at signals and so on, obviously that would be a bit more logic for not a lot of gain, but it would be strictly more safe than relying on the GC not removing that class too early (although some comments in GLib/GObject seem to indicate that the instantiated classes shouldn't disappear again, "static types").