service-dbus: Fixed the test to check also instantiated services.
With git commit 3bb8d59f7a9008bb3fe118c96ea6d1fd82aa5329, the provider supports
instantiated services (like getty@tty1.service or user@1000.service).
The test did not expect that and failed with "Items in the second set but not
the first: u'getty@tty1.service'". Let's fix the test and check the instances.