Replace test/support/github_formatter with rspec-github gem

[issyl0]

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• This was a TODO in test/support/github_formatter.rb and the PR it referenced had been merged.
• Will mark as not-draft when CI passes - Xcode is updating on my local machine and taking forever.

[BrewTestBot]

Review period will end on 2020-12-23 at 18:14:40 UTC.

[BrewTestBot]

Review period ended.