RFC: Allow marking unreliable tests as flaky.

[Esme Povirk (she/they)]

It might be better to use something like broken() to encourage more
precision in marking the failures. For example, if something fails
with ACCESS_DENIED sometimes, we'd still like to know if it starts
failing with FILE_NOT_FOUND. That does make it trickier to add flaky
tests to the summary line, though. I guess you could set it up so that
if flaky() is called with non-zero and the next ok() succeeds, it's
marked as flaky.

Having the failures in the summary line also opens up the option of
handling them by retrying, and can be implemented without making that
decision first. Of course, retrying leaves open the possibility that
you will be unlucky and all the retries will also fail. Whether that's
worth it would probably depend on how often a regression on a flaky
test slips through and causes the tests to fail for Alexandre.