gh-109162: Refactor libregrtest WorkerJob (#109171)

* Rename --worker-args command line option to --worker-json.
* Rename _parse_worker_args() to _parse_worker_json().
* WorkerJob:

  * Add runtests attribute
  * Remove test_name and rerun attribute

* Rename run_test_in_subprocess() to create_worker_process().
* Rename run_tests_worker() to worker_process().
* create_worker_process() uses json.dump(): write directly JSON to
  stdout.
* Convert MultiprocessResult to a frozen dataclass.
* Rename RunTests.match_tests to RunTests.match_tests_dict.