gh-109162: libregrtest: rename runtest_mp.py to run_workers.py (#109248)

* Rename runtest_mp.py to run_workers.py
* Move exit_timeout() and temp_cwd() context managers from
  Regrtest.main() to Regrtest.run_tests(). Actions like --list-tests
  or --list-cases don't need these protections.
* Regrtest: remove selected and tests attributes. Pass 'selected' to
  list_tests(), list_cases() and run_tests(). display_result() now
  expects a TestTuple, instead of TestList.
* Rename setup_tests() to setup_process() and rename setup_support()
  to setup_tests().
* Move _adjust_resource_limits() to utils and rename it to
  adjust_rlimit_nofile().
* Move replace_stdout() to utils.
* Fix RunTests.verbose type: it's an int.