PyGObject has established the assumption that the destruction of a GLib.Source Python object does not destroy the actual GSource, as shown in TestSource.setup_timeout(), TestTimeout.test504337(), and https://bugzilla.gnome.org/show_bug.cgi?id=504337.

So we need to explicitly destroy our MySource and Idle objects after using 
them, as their callbacks always return True and we do not want them to spill over into other tests.

Also, that test has a bad bug:

        assert self.pos >= 0 and idle.count >= 0

This does not actually assert anything. We need to compare self.pos to > 0, as we want to enforce that MySources' callback was called at least once.