test: runner support for flaky tests

Adding --flaky-tests option, to allow regarding flaky tests failures
as non-fatal.

Currently only observed by the TapProgressIndicator, which will
add a # TODO directive to tests classified as flaky. According to the
TAP specification, the test harness is supposed to treat failures
that have a # TODO directive as non-fatal.

PR-URL: https://github.com/joyent/node/pull/25686


Reviewed-By: Julien Gilli <julien.gilli@joyent.com>