test-lib: make '--stress' more bisect-friendly

Let's suppose that a test somehow becomes flaky between 'master' and
'pu', and tends to fail within the first 50 repetitions when run with
'--stress'.  In such a case we could use 'git bisect' to find the
culprit: if the test script fails with '--stress', then the commit is
definitely bad, but if it survives, say, 300 repetitions, then we could
consider it good with reasonable confidence.

Unfortunately, all this could only be done manually, because
'--stress' would run the test script repeatedly for all eternity on a
good commit, and it would exit with success even when it found a
failure on a bad commit.

So let's make '--stress' usable with 'git bisect run':

  - Make it exit with failure if a failure is found.

  - Add the '--stress-limit=<N>' option to repeat the test script
    at most N times in each of the parallel jobs, and exit with
    success when the limit is reached.

And then we could simply run something like:

  $ git bisect start origin/pu master
  $ git bisect run sh -c 'make && cd t &&
                          ./t1234-foo.sh --stress --stress-limit=300'

Sure, as a brand new feature it won't be any useful right now, but in
a release or three most cooking topics will already contain this, so
we could automatically bisect at least newly introduced flakiness.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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