Add tests for idle_add()/timeout_add with and without user data
Add tests for idle_add()/timeout_add with and without user data

This is implicitly spread over various test cases, but let's test it explicitly
to ensure that the behaviour stays consistent when moving this to GI.

davibe/pygobject,pexip/pygobject,GNOME/pygobject,GNOME/pygobject,davibe/pygobject,sfeltman/pygobject,pexip/pygobject,sfeltman/pygobject,GNOME/pygobject,pexip/pygobject,davibe/pygobject,sfeltman/pygobject,davibe/pygobject