Currently if an Android plugin has no native unit tests, we silently skip it. This means it's easy to accidentally add a plugin without a test harness, or to accidentally break the running of unit tests (e.g., if their location changes but the script isn't updated).

Once #83358 is fixed, or at least further along, we should make it an error for a plugin that has an Android implementation in the package to not run any unit tests, and explicitly --exclude any we still need to have skipped. This makes it much harder to silently do the wrong thing, and is consistent with changes that have been recently made to other test commands in the tooling.