Improve reproducibility of test runs.

Add random seed to run-tests.py, using either a user supplied
value or a random number generated by random.SystemRandom().
This same random seed is passed to all test cases, making sure
that we can easily reproduce test failures that depend on
random numbers (i.e. bugs related to our handwritten ASLR).

Also fix all uses of rand() to make use of our RNG class
instead.

R=machenbach@chromium.org

Review URL: https://codereview.chromium.org/231443002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20637 ce2b1a6d-e550-0410-aec6-3dcde31c8c00