[3.5] bpo-30383: Backport regrtest and test_regrtest enhancements from master to...

[3.5] bpo-30383: Backport regrtest and test_regrtest enhancements from master to 3.5 (#2279)

* bpo-30383: regrtest: prepend testdir to sys.path

* bpo-30383: Backport test_regrtest

* regrtest: rename --slow option to --slowest

The old --slow syntax is still accepted.

* regrtest: add a single oneliner summary

Example: "Tests result: SUCCESS"

* test_regrtest: add test_coverage()

regrtest now also displays the number of successful tests when
coverage is used.

* test_regrtest: add test_crashed()

Handle correctly crashing test: account the crash has a failed test,
but continue to run other tests.

* regrtest: backport --list-tests feature

* regrtest: backport --fromfile enhancements

* regrtest: backport displaying progress enhancements

* test_regrtest: backport test_randseed()

* regrtest: Fix --coverage on Windows

Don't ignore any directory anymore. Change backported from master.