If we can't get on the session bus, just behave like a normal non-unique
application.
This turns out to be remarkably easy to implement and lets us avoid
adding a 'dummy' backend.
Add a test for this case as well.
Idea from Zachary Dovel.
https://bugzilla.gnome.org/show_bug.cgi?id=651997
The GApplication test case tried to fork() while using GMainLoop,
causing problems. Avoid doing that by splitting the child process into
a separate program and spawning it in the usual way.
https://bugzilla.gnome.org/show_bug.cgi?id=658999