Clears screen during watch mode correctly.

[marr]

Summary
This change is to prevent the screen from clearing when building for Continuous Integration systems. While the --watch flag typically wouldn't be used on a CI system, this change gives us a hook into disabling the clear feature that can prevent other processes' output from being seen during a typical development workflow.

Test plan
I was able to verify this worked using the CI=true jest --watchAll npm-script in my project. Without the CI value set, the standard behavior of the screen clearing between watch runs, occurs.

Fixes #2959.

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[cpojer]

Thank you so much for fixing this! I'm sorry for the thrash on that issue, my mistake and I apologize.

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