This fixes #111 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.