gh-109162: Refactor libregrtest.runtest (#109172)

* Rename runtest() to run_single_test().
* Pass runtests to run_single_test().
* Add type annotation to Regrtest attributes. Add missing attributes
  to Namespace.
* Add attributes to Regrtest and RunTests:

  * fail_fast
  * ignore_tests
  * match_tests
  * output_on_failure
  * pgo
  * pgo_extended
  * timeout

* Get pgo from 'runtests', rather than from 'ns'.
* Remove WorkerJob.match_tests.
* setup_support() now gets pgo_extended from runtests.
* save_env(): change parameter order, pass test_name first.
* Add setup_test_dir() function.
* Pass runtests to setup_tests().