Add script for running stress tests.

[robertnishihara]

This should make it possible to run stress tests just by doing ./test/stress_tests/run_stress_tests.sh.

TODO before merging:

• Change from 2 nodes to 200.
• [optional] Make it easy to test different commits/branches/repos by passing in a different argument to the script.
• Add more tests.

[robertnishihara]

cc @stephanie-wang @ericl @richardliaw there are no interesting tests here yet, but this is an outline of how we can do stress testing.

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[robertnishihara]

This needs to be ray.init(redis_address="localhost:6379") in order to attach to the cluster.

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