gh-109162: libregrtest: remove WorkerJob class (#109204)

* Add attributes to Regrtest and RunTests:

  * gc_threshold
  * memory_limit
  * python_cmd
  * use_resources

* Remove WorkerJob class. Add as_json() and from_json() methods to
  RunTests. A worker process now only uses RunTests for all
  parameters.
* Add tests on support.set_memlimit() in test_support. Create
  _parse_memlimit() and also adds tests on it.
* Remove 'ns' parameter from runtest.py.