[2.7] bpo-33873: Backport regrtest from master by vstinner · Pull Request #7936 · python/cpython

…H-7686)

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

(cherry picked from commit 4ffe9c2)

regrtest: Add warning when using less than 3 warmup runs like -R 1:3.

(cherry picked from commit cac4fef)

…rameters (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 58ed730)