[3.5] bpo-30383: Backport regrtest and test_regrtest enhancements from master to 3.5 by vstinner · Pull Request #2279 · python/cpython

The old --slow syntax is still accepted.

Example: "Tests result: SUCCESS"

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

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