test-lib: add the '--stress' option to run a test repeatedly under load

Unfortunately, we have a few flaky tests, whose failures tend to be
hard to reproduce.  We've found that the best we can do to reproduce
such a failure is to run the test script repeatedly while the machine
is under load, and wait in the hope that the load creates enough
variance in the timing of the test's commands that a failure is
evenually triggered.  I have a command to do that, and I noticed that
two other contributors have rolled their own scripts to do the same,
all choosing slightly different approaches.

To help reproduce failures in flaky tests, introduce the '--stress'
option to run a test script repeatedly in multiple parallel jobs until
one of them fails, thereby using the test script itself to increase
the load on the machine.

The number of parallel jobs is determined by, in order of precedence:
the number specified as '--stress=<N>', or the value of the
GIT_TEST_STRESS_LOAD environment variable, or twice the number of
available processors (as reported by the 'getconf' utility), or 8.

Make '--stress' imply '--verbose -x --immediate' to get the most
information about rare failures; there is really no point in spending
all the extra effort to reproduce such a failure, and then not know
which command failed and why.

To prevent the several parallel invocations of the same test from
interfering with each other:

  - Include the parallel job's number in the name of the trash
    directory and the various output files under 't/test-results/' as
    a '.stress-<Nr>' suffix.

  - Add the parallel job's number to the port number specified by the
    user or to the test number, so even tests involving daemons
    listening on a TCP socket can be stressed.

  - Redirect each parallel test run's verbose output to
    't/test-results/$TEST_NAME.stress-<nr>.out', because dumping the
    output of several parallel running tests to the terminal would
    create a big ugly mess.

For convenience, print the output of the failed test job at the end,
and rename its trash directory to end with the '.stress-failed'
suffix, so it's easy to find in a predictable path (OTOH, all absolute
paths recorded in the trash directory become invalid; we'll see
whether this causes any issues in practice).  If, in an unlikely case,
more than one jobs were to fail nearly at the same time, then print
the output of all failed jobs, and rename the trash directory of only
the last one (i.e. with the highest job number), as it is the trash
directory of the test whose output will be at the bottom of the user's
terminal.

Based on Jeff King's 'stress' script.

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

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