gh-109162: libregrtest: add TestResults class (#109208)

* Add TestResults class.
* Move Regrtest methods to TestResults:

  * accumulate_result(): now takes a RunTests parameter
  * get_executed()
  * no_tests_run()

* Add methods to TestResults:

  * add_junit()
  * display_result()
  * display_summary()
  * need_rerun()
  * prepare_rerun()
  * write_junit()

* Rename 'need_rerun' attribute to 'bad_results'.
* Rename 'total_stats' attribute to 'stats'.

Author: vstinner