Backport manually regrtest enhancements from master:

* No longer clear filters, like --match, to re-run failed tests in
  verbose mode (-w option).
* Tests result: always indicate if tests have been interrupted.
* Enhance tests summary
* After failing tests are re-run, display again the summary.
* Add environment_altered to test.support, but it's currently unused
* regrtest: count also ENV_CHANGED as failures
* regrtest: Enhance format_duration()
* Replace test_support with support