If tracker-miners is configured with the `-Dtracker_core=subproject` option,
it should use GSettings schemas from the ./subprojects/tracker build
tree. We weren't doing that, which meant that if you ran `dnf remove
tracker` on your test image and then ran the test, you would see
failures like this in the functional-tests:

    (process:8450): GLib-GIO-ERROR **: 09:43:34.625: Settings schema 'org.freedesktop.Tracker.Store' is not installed

This fix is part of tracker#24