Now that we're not calling g_object_run_dispose() indirectly in
g_test_dbus_down() (see commit "Revert "gtestdbus: Properly close server
connections""), the test gdbus-connection-loss is failing with the
message "Bail out! GLib-GIO-FATAL-WARNING: Weak notify timeout, object
ref_count=1". This is because we're holding a reference to the singleton
connection object while calling session_bus_down() in the test's main().
So then we end up waiting for 30 seconds in
_g_object_unref_and_wait_weak_notify() for the GWeakNotify to be
triggered, which never happens.

The fix is to unref the connection before calling session_bus_down().
This is consistent with how other tests work, and is safe because the
only method called on the connection has already errored out, as
asserted by the test.