gh-109162: Refactor libregrtest.runtest_mp (#109205)

* Add attributes to Regrtest and RunTests:

  * fail_env_changed
  * num_workers

* Rename MultiprocessTestRunner to RunWorkers. Add num_workers
  parameters to RunWorkers constructor. Remove RunWorkers.ns
  attribute.
* Rename TestWorkerProcess to WorkerThread.
* get_running() now returns a string like: "running (...): ...".
* Regrtest.action_run_tests() now selects the number of worker
  processes, instead of the command line parser.