I dunno. I don't miss the 1-minute incremental builds. (Maybe they've improved since I left, though.)
BTW Forge is not just the test runner, but the thing that runs all build tasks, farmed out to all servers. Blaze interprets the build language and does dependency tree analysis but then hands off the tasks to Forge. Blaze has been (partially) open sourced: https://bazel.build/
