racetest: Review

- Factor common logic to spawn and run group of test threads into
  TestGroup. This way the extra checks and robustness improvements, that
  Dieter just added to check_race_external_invalidate_vs_disconnect,
  become available to all tests in racetest module.

  Rework moved code so that nwork is not fixed beforehand and test threads
  can be added dynamically.

- fix waiting logic in Finished:
  * on py2 Condition.wait does not return True/False as it does on py3 -
    we need to manually inspect the state.
  * fix race for when wait is called with already met condition:
    previously in such case it was waiting indefinitely and reporting
    failure on timeout

- rename Finished to well-established concept of WaitGroup and adjust
  its interface accordingly (see
  https://pkg.go.dev/sync#WaitGroup,
  https://lab.nexedi.com/nexedi/pygolang/blob/master/golang/sync.py and
  https://lab.nexedi.com/nexedi/pygolang/blob/39dde7eb/golang/sync.cpp#L153-200)

- no need to wrap try/except with additional try/finally, as
  try/except/finally works out of the box.