formula_creator: test GitHub repo call for archive|releases URLs

[abitrolly]

• 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.
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• Have you successfully run brew tests with your changes locally?

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Adds tests for GitHub parsing section, to ensure that PRs that touch it like #20025 don't break things.

[MikeMcQuaid]

Thanks!