build_planner: Add slim version of eligible CQ builders to buildsToRun.

Add utility to select a slim version of the build to run if all repos
are slim elibible and the builder is not configured to perform HW or VM
tests in CQ.

BUG=chromium:1094321
TEST=./run_tests.sh

Change-Id: I721a2652d9d968ec08732692b3e67fb92a2866e6
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/go/+/2297800
Tested-by: Navil Perez <navil@google.com>
Commit-Queue: Navil Perez <navil@google.com>
Reviewed-by: Sean Abraham <seanabraham@chromium.org>
3 files changed