test-bot: move to tap.

[MikeMcQuaid]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew tests with your changes locally?

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Migrate to https://github.com/Homebrew/homebrew-test-bot to be able to work on this with a different, non-tagged workflow.

[zmwangx]

[ilovezfs]

Error: uninitialized constant Homebrew::Test::Diagnostic

[ilovezfs]

ah nevermind... figured out what was up