test: runner should return 0 on flaky tests

Make the test runner return a 0 exit code when only
flaky tests fail and --flaky-tests=dontcare is specified.

Ported from https://github.com/joyent/node/commit/a9b642cf5b539e1b86698465d3d0b941d84c5352

PR-URL: https://github.com/nodejs/node/pull/2424


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: João Reis <reis@janeasystems.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>