bpo-30283: Backport regrtest features from master to 2.7 (#1516)

bpo-30283: Backport regrtest features from master to 2.7 (#1516)

* regrtest: add --slowest alias to --slow

* make buildbottest: add --slowest option

* regrtest: add "- " prefix to --slowest output

* regrtest: Fix an outdated comment

* regrtest: replace PermissionError

Replace PermissionError with OSError and check on exc.errno.
PermissionError was added to Python 3.3.

* regrtest: add -3 -tt options to run Python scripts

* regrtest: backport --list-tests option

* regrtest: backport "Tests result: xxx" summary

* regrtest: backport total duration

* regrtest: add timestamp to the progress

* regrtest: describe previous test state

* Add the state of the test: passed, failed, etc.
* If a test took longer than 30 seconds, log its execution time

* regrtest: -jN logs running workers

* regrtest: mention if tests are run in parallel

* regrtest: parallel mode is more verbose during wait

Display running tests every 30 seconds if no test completed in the
meanwhile.

* test_regrtest: fix typo in SubprocessRun