Add RSpec formatter for Github Actions.

[reitermarkus]

• Have you followed the guidelines in our Contributing document?
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• 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 style with your changes locally?
• Have you successfully run brew tests with your changes locally?

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Output annotations for test failures.