functional-tests: Don't require Tracker to be installed in the host OS when -Dtracker_core=subproject is passed

Currently, our CI image contains the 'tracker' package. If this is removed, the functional-tests fail with this error:

(process:4024): GLib-GIO-ERROR **: 15:39:35.114: Settings schema 'org.freedesktop.Tracker.Store' is not installed

We do set GSETTINGS_SCHEMA_DIR when running the functional-tests, but this only contains the schemas from tracker-miners.git, not tracker.git, hence the error if these schemas are not available in /usr.