If "tracker3 status" is run while tracker-extract-3 is running, it
looks enough of a miner to poke at it, but we get:

(tracker status:53148): Tracker-CRITICAL **: 12:26:14.447: No D-Bus proxy found for miner 'org.freedesktop.Tracker3.Miner.Extract'

Since the service is not listed in our special directory. Make
TrackerMinerManager skip over unlisted matches, so we don't issue
this warning.

However, might be nice to carry the status/timing information from
tracker-extract-3 to "tracker3 status". Probably carried over by
tracker-miner-fs-3.