Allow per-builder blacklisting of unittests.

Add a feature for per-builder blacklisting of unit tests. This can be
used to help work around issues where a unit test is only broken on a
particular builder.

In the case of the update engine unit tests, they are unable to run in
parallel so it means that if two builders run them at once on the same
machine, they break. I worked around this issue in this by disabling them
from running on parrot-pre-cq. They still run on stout-pre-cq and on all
other builders.

TEST=Remote trybot run.

Change-Id: Idf7abddd5392d45f0f049302ed71c384c8483173
Commit-Queue: David James <>
Reviewed-by: David James <>
Tested-by: David James <>
4 files changed