bpo-33873: Backport regrtest from master to 3.7 (GH-7935)

* bpo-33718: regrtest: use format_duration() to display failed tests (GH-7686)

* Enhance also format_duration(): work on integers and rounds towards
  +infinity (math.ceil).
* Write unit tests on format_duration()

(cherry picked from commit 4ffe9c2b251f6e027b26250b7a2618e78d4edd22)

* bpo-33873: regrtest: Add warning on -R 1:3 (GH-7736)

regrtest: Add warning when using less than 3 warmup runs like -R 1:3.
(cherry picked from commit cac4fef8860e66a9da67d09762f5b614b9471a12)

* bpo-33873: Fix bug in `runtest.py` and add checks for invalid `-R` parameters (GH-7735)

Fix bug in `Lib/test/libregrtest/runtest.py` that makes running tests an extra time than the specified number of runs.

Add check for invalid --huntrleaks/-R parameters.

(cherry picked from commit 58ed7307ea0b5c5aa052291ebc3030f314f938d8)
diff --git a/Lib/test/libregrtest/runtest.py b/Lib/test/libregrtest/runtest.py
index 12bf422..3e1afd4 100644
--- a/Lib/test/libregrtest/runtest.py
+++ b/Lib/test/libregrtest/runtest.py
@@ -173,9 +173,10 @@
                     if loader.errors:
                         raise Exception("errors while loading tests")
                     support.run_unittest(tests)
-            test_runner()
             if ns.huntrleaks:
                 refleak = dash_R(the_module, test, test_runner, ns.huntrleaks)
+            else:
+                test_runner()
             test_time = time.time() - start_time
         post_test_cleanup()
     except support.ResourceDenied as msg: