builder_config.proto: enable specifying unit test packages

Enables to ability to specify which unit tests to run within a builder
config. This will be used in the code coverage builders to only build
coverage for specific packages.

BUG=b:161462144
TEST=None

Change-Id: I7b34fdac72c13a58f0be372a099f05d92f2fb72d
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/proto/+/2303892
Tested-by: Brendan Hansknecht <bhansknecht@chromium.org>
Reviewed-by: David Burger <dburger@chromium.org>
Commit-Queue: Brendan Hansknecht <bhansknecht@chromium.org>
2 files changed