Randomize the order of test cases passed from junit_run.py to junit-runner task

[ericzundel]

@cheister found a couple of flaky tests in our repo in the latest upgrade to 1.2.0. There was a difference between 1.1.0 in the order that test cases were passed to junit-runner.jar that exposed flaky tests.

This gave us an idea that we should be able to pass a flag to randomize the tests and print out the seed so that we can try running tests repeatedly in different order. If they fail, we can re-run them with the same seed in order to debug what's going wrong.

./pants test tests/src/java/com/squareup/foo:: --test-junit-randomize
...
  [test.junit]
     Randomizing tests with seed 1234567

and then, if that fails, run something like:

./pants test  tests/src/java/com/squareup/foo:: --test-junit-randomize --test-junit-random-seed=1234567