tests: Add test for #690400

Test that removing a notify-callback from the callback itself works.

https://bugzilla.gnome.org/show_bug.cgi?id=690400
Survive ServiceProxy callback-list changes from callbacks

Make notify-callback code more robust, so that removing a
notify-callback from a notify-callback works.

https://bugzilla.gnome.org/show_bug.cgi?id=690400
tests: refactor main loop handling to one function