Each builder now uses a dict to specify tests with a 'tests' key. This
will allow adding other builder-specific keys in follow ups.

Prepared on infra side by:
https://crrev.com/c/1013713

Bug: chromium:830557
Change-Id: I1dd799cdfcc029ab22de813b7869abe982047e0f
Reviewed-on: https://chromium-review.googlesource.com/1013979Reviewed-by: Sergiy Byelozyorov <sergiyb@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52656}