test: add --repeat option to tools/test.py

I often want to run a test many times to see if a failure
can be recreated and I believe this is a common
use case.  We even have this job in the CI
https://ci.nodejs.org/job/node-stress-single-test/configure
but often you want to run it on a specific machine.

This patch adds the --repeat option so that
you can repeat the selected set of tests a
number of times. Given existing options
in test.py this will allow you to run
one or more tests for the number of
repeats specified. For example:

tools/test.py -j8 --repeat 1000 parallel/test-process-exec-argv

runs the test-process-exec-argv test 1000 times,
running 8 copies in parallel

tools/test.py --repeat 2

would run the entire test suite twice.

PR-URL: https://github.com/nodejs/node/pull/6700


Reviewed-By: Ben Noorhduis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Fedor Indutny <fedor@indutny.com>
Reviewed-By: thefourtheye - Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: joaocgreis - João Reis <reis@janeasystems.com>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>