regrtest computes statistics (#108793)

test_netrc, test_pep646_syntax and test_xml_etree now return results
in the test_main() function.

Changes:

* Rewrite TestResult as a dataclass with a new State class.
* Add test.support.TestStats class and Regrtest.stats_dict attribute.
* libregrtest.runtest functions now modify a TestResult instance
  in-place.
* libregrtest summary lists the number of run tests and skipped
  tests, and denied resources.
* Add TestResult.has_meaningful_duration() method.
* Compute TestResult duration in the upper function.
* Use time.perf_counter() instead of time.monotonic().
* Regrtest: rename 'resource_denieds' attribute to 'resource_denied'.
* Rename CHILD_ERROR to MULTIPROCESSING_ERROR.
* Use match/case syntadx to have different code depending on the
  test state.

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>