Draft: [tracker3] Add test for tag-manager

As part of !553 (merged) I added a test for the 'starred files' feature.

This is a separate MR to !553 (merged) as it currently depends on an unmerged feature of Tracker 3. It also adds quite some Tracker-specific code to the test suite, I would hope for this to be provided in a new library from tracker-miners in future but I don't think this will happen for 3.0.