gh-109162: libregrtest: add single.py and result.py (#109243)

* Add single.py and result.py files.
* Rename runtest.py to runtests.py.
* Move run_single_test() function and its helper functions to
  single.py.
* Move remove_testfn(), abs_module_name() and normalize_test_name()
  to utils.py.
* Move setup_support() to setup.py.
* Move type hints like TestName to utils.py.
* Rename runtest.py to runtests.py.