* Add Helper base class.

* New setup behaviour: start() runs the process and blocks until it is
  available on the bus. connect() and wait() are removed.

* A new process is always explicitly spawned, instead of relying on DBus
  activation. This allows passing environment variables to the tracker
  processes being tested, regardless of whether they are being run in
  the session dbus or a nested bus from test-helper.sh.

* common/utils/system.py: use Helper instead of our own LifeCycle
  classes because they now do the same thing.

* StoreHelper: don't try to respawn the tracker-store unless it actually
  has disappeared from the bus - other exceptions are now passed to the
  caller.

All tests fixed for new behaviour.