[2.7] bpo-33718: Update regrtest from master (GH-7402)

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

Lib/test/support/__init__.py

@@ -1671,6 +1671,14 @@ def run_doctest(module, verbosity=None):
 #=======================================================================
 # Threading support to prevent reporting refleaks when running regrtest.py -R
 
+# Flag used by saved_test_environment of test.libregrtest.save_env,
+# to check if a test modified the environment. The flag should be set to False
+# before running a new test.
+#
+# For example, threading_cleanup() sets the flag is the function fails
+# to cleanup threads.
+environment_altered = False
+
 # NOTE: we use thread._count() rather than threading.enumerate() (or the
 # moral equivalent thereof) because a threading.Thread object is still alive
 # until its __bootstrap() method has returned, even after it has been