Rework the tracker-sandbox utility (again)

This fixes #111 (closed) and also simplifies the instructions in the README for running Tracker from the build tree.

The sandbox utility is now executed from the top directory by running:

python3 -m utils.trackertestutils

Previously, due to importing stuff from trackertestutils. you needed to run it from the utils/ directory or set PYTHONPATH appropriately.

Additionally, tracker-miners.git will ship a 'run-uninstalled' script to provide convenient access to the sandbox script and allow running Tracker from the build tree.

Depends on !130 (merged)