gh-109974: Fix more threading lock_tests race conditions (#110089)

* Add context manager on Bunch class.
* Bunch now catchs exceptions on executed functions and re-raise them
  at __exit__() as an ExceptionGroup.
* Rewrite BarrierProxy.test_default_timeout(). Use a single thread.
  Only check that barrier.wait() blocks for at least default timeout
  seconds.
* test_with(): inline _with() function.

Author: vstinner