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This CL implements gn based unittest filtering. The files in the rts_exclude_file are manually selected for this example, but will later be populated using the smart logic in rts-chromium.
These two cases (source_set and test template) cover most of the test files. I haven't seen any other templates with unittests, but even if there are some this would be a great start. This also doesn't cover webtests, but I think there's enough exclusion power here to save a lot of compute.
This also adds a linux-rts builder to the mb config to prepare the experimental builder.

Bug: 1145216
Change-Id: I7343249f3df0739e39cfe351e5714d7978059d86
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2551276
Commit-Queue: Gregory Guterman <guterman@google.com>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Reviewed-by: Nodir Turakulov <nodir@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841248}