Fix functional-tests when Tracker is installed

We need to honour the TRACKER_TEST_DOMAIN_ONTOLOGY_RULE whether or not Tracker is installed. It's not just a fallback for the default domain rule, it also ensures that the uninstalled versions of the ontologies are used.

I wondered if we could solve this problem by starting the test instances of the miners with --domain-ontology set. However, this causes them to exit as soon as they are started because nothing owns the name org.freedesktop on the message bus. (Hence the other commit on this branch :)