formula_desc_cop: Whitelist "etc." and fix the full stop autocorrect

[issyl0]

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As per #2795 (comment), here's a PR to whitelist "etc." only in the full stop format_desc_cop, and fix the autocorrect so it works with quotes at the end of the line too. I ran a brew audit --strict and it stopped complaining about formulae with etc. at the end.

[MikeMcQuaid]

Looks good, thanks @issyl0! Would you mind writing some tests to check these work as expected? Thanks again!

[MikeMcQuaid]

@issyl0 Any news on that? If you'd rather not: no worries.

[issyl0]

Sorry @MikeMcQuaid, this week has been stupidly busy and I'm unlikely to get time to do this soon. Sorry for leaving this hanging too. Also, from what I can see, the existing tests always test that violations are reported, not that they're ignored, if that makes any sense, and negating that for this one case feels odd.

[MikeMcQuaid]

Will merge without tests this time but please write some in future. Thanks @issyl0!