rubocop: Enable RSpec/RepeatedExampleGroupDescription cop & fix offenses

[issyl0]

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• Inspired by my other RuboCop PR on Friday, here's another TODO dealt with.

[BrewTestBot]

Review period will end on 2021-02-23 at 00:00:00 UTC.

[Rylan12]

Looks good. One suggestion that isn't crucial

[Rylan12]

Suggested change

	it {
	it "returns a file with a `.dmg` extension" {

Might be a good idea to add some sort of description like this to the its to it conversions.

Without a description:

CurlDownloadStrategy#cached_location with a file name trailing the URL path is expected to eq ".dmg"

With a description:

CurlDownloadStrategy#cached_location with a file name trailing the URL path returns a file with a `.dmg` extension

[issyl0]

I mostly didn't add descriptions as the other it {s that I moved down didn't have any. 🤷🏻

[MikeMcQuaid]

Nice work @issyl0!

[BrewTestBot]

Review period ended.