By removing the cached global proxy in gdocumentportal.c, we can
re-enable the checks for proper shutdown of the session bus connection
in the dbus-appinfo.c test.
We can't use session_bus_down() in the test since gdocumentportal.c
holds a reference to the session bus connection, preventing it from
being finalised.
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
