Rework the tracker-sandbox utility (again)

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.
1 job for sam/sandbox-rewrite-2 in 35 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
failed #427484
test-fedora-latest

00:00:35

 
Name Stage Failure
failed
test-fedora-latest Test
Program /builds/GNOME/tracker/utils/sandbox/tracker-sandbox.py found: NO

tests/functional-tests/ipc/meson.build:6:0: ERROR: Program(s) ['/builds/GNOME/tracker/utils/sandbox/tracker-sandbox.py'] not found or not executable

A full log can be found at /builds/GNOME/tracker/build/meson-logs/meson-log.txt
Uploading artifacts...
WARNING: build/meson-logs/testlog.txt: no matching files
ERROR: No files to upload
ERROR: Job failed: exit code 1