https://bugs.webkit.org/show_bug.cgi?id=124550

Reviewed by Darin Adler.

This might not be the case for all hosts, so this patch implements an alternate "backend" 
for run-jsc-stress-tests to use normal shell commands rather than Makefiles. To remain at 
least somewhat competitive with the make-based test runner, the shell backend uses subshells
run in the background to allow tests to run in parallel. Since the concurrency primitives 
in shell scripting are rather coarse, the overhead of this parallelism is higher than that 
of the make-based runner.

* Scripts/jsc-stress-test-helpers/shell-runner.sh: Added. This is the runner that is copied into
the bundle and controls all of the parallel aspects of the shell-based test runner.
* Scripts/run-jsc-stress-tests:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@160016 268f45cc-cd09-0410-ab3c-d52691b4dbfc