Add mocha-fgrep flag to run_test_suite.js
Add mocha-fgrep flag to run_test_suite.js

This CL adds the ability to filter the tests via Mocha's `fgrep` flag,
giving us the ability to run a subset of the tests if we wish in the
future.

Fixed: 1249000
Change-Id: I3379f6f0f60fc97d8af6b019761cfb31dda8afd2
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/3162297
Reviewed-by: Paul Lewis <8915f5d5c39a08c5a3e8a8d7314756e4580816f9@chromium.org>
Commit-Queue: Jack Franklin <993fcadce4d04090da2fefd557a0995e7966c8d5@chromium.org>

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