gh-109162: Refactor libregrtest.Regrtest (#109206)

* Add type hint types: TestName, StrPath, StrJSON.
* Add attributes to Regrtest:

  * cmdline_args
  * coverage
  * coverage_dir
  * fail_rerun
  * next_single_filename
  * print_slowest
  * tmp_dir
  * want_rerun
  * want_run_leaks

* Remove Regrtest.ns attribute.
* Rename Regrtest methods:

  * cleanup() => cleanup_temp_dir()
  * create_temp_dir() => make_temp_dir()
  * set_temp_dir() => select_temp_dir()

* Convert Regrtest methods to static methods:

  * cleanup_temp_dir()
  * display_header()
  * fix_umask()
  * get_rerun_match_tests()
  * list_tests()
  * make_temp_dir()
  * select_temp_dir()

* Remove display_sanitizers() method: move code into
  display_header().
* Rename 'test_cwd' variable to 'work_dir'.