WIP: Isolate the tests from any real user data

This uses the trackertestutils library to set up a temporary directory for XDG_DATA_HOME and XDG_CACHE_HOME data, and spawn private Tracker daemons which operate from this temporary directory. This means the app under test will no longer change behaviour depending on data in the HOME directory of the user running the test.

Marked as WIP because it depends on this tracker.git MR: tracker!147 (merged)