'brew tests --changed' supports changed test files

[apainintheneck]

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This updates the brew tests --changed option to support changed test files. Before it would only run tests that correspond to changed library files but wouldn't run tests on changed test files. It also ignores Ruby files that lack the *_spec.rb ending in the test folder like fixtures.

The reason that I made it another method was because the linter was yelling at me about a long block and, to be honest, I agree that it's more readable.

[MikeMcQuaid]

Makes sense, thanks again @apainintheneck!