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>