Add "spray" argument for Replay and Reducer

[pyoor]

Often times when trying to replay intermittent test cases, it can be advantageous to open the same testcase in multiple tabs concurrently to increase the chance of triggering the bug. We could easily add this feature to the harness.

I propose that we add a --spray argument which accepts an int defining the number of tabs to open the testcase in.

[areuu]

Why "open the same test case in multiple tabs" can "increase the chance of triggering the bug"? How about trying to replay with headless mode? Or modify some browser source code to reduce?

[pyoor]

This is one of the tricks we use for reproducing intermittent testcases. There's a number of reasons why this can work for some testcases. Running multiple instances of the same testcase can increase memory pressure and CPU usage. Further, testcases which trigger due to use of global objects or shared memory may also be more likely to trigger when running multiple instances of the testcase.

Also, just to clarify -- you can run any testcase in "headless" mode via --xvfb