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This CL adds unit tests for build/chromeos/test_runner.py.

Bug: 1158590
Change-Id: I448fe91423e6bcb511a11f27f6409232909875c2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2605401Reviewed-by: Ben Pastene <bpastene@chromium.org>
Commit-Queue: Yuke Liao <liaoyuke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840432}