This is to avoid having again the subtil bug in dbus-appinfo.c:
session_bus_down() was called before g_test_run() so the test was
running on the user's dbus session.
https://bugzilla.gnome.org/show_bug.cgi?id=697348
Add a fairly realistic testcase that ensures that GDesktopAppInfo with
DBusActivatable=true can successfully talk to GApplication for a variety
of purposes.
https://bugzilla.gnome.org/show_bug.cgi?id=699259