Open source internal test runner

Through a few conversations with Googlers I learned that internally a different test runner is used with rules_apple than the one that is publicly supported. I think because of this the open source version has a lack of testing which can cause [issues](https://github.com/bazelbuild/rules_apple/issues/490) that we likely wouldn't hit if the community shared the test runner that is currently used by Google. It would be great if that runner could be open sourced so that we could all contribute to it and improve a single tool, rather than being required to maintain our own test runners.

Start by reviewing the public test-runner setup in rules_apple and the related issue #490. The issue does not name files, tests, or an entry point, so first determine what internal runner is being requested and what can be published. Done would mean the requested runner is open sourced for community contribution.

Written by the indexing model from the issue text.