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This CL changes the tests for generate_buildbot_json.py to capture all
printed lines. It requires tests to assert about the contents of these
printed lines, and then clear them.

It also adds a --verbose flag to the main script, which is passed when
checking the configs.

Also fixes a test for swarming mixins and how they interact with certain
classes of tests.

Bug: 886993
Change-Id: I047d74689d72f35bc7d19c8da338c070a8a5d9a5
Reviewed-on: https://chromium-review.googlesource.com/1237239
Commit-Queue: Stephen Martinis <martiniss@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593015}