Refactor specs to use webmock

We approved Improve error when PAT doesn't have required ac... (!35 - merged) • Félix Veillette-Potvin • 17.7 but recognised that the specs are failing and written in a hard-to-maintain way. @fvpotvin offered to fix them in a followup MR:

I've been fighting trying to get the tests to pass 😓 The way they are mocked right now doesn't take into account that it takes the code attribute of response (its always empty arrays right now).

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I'll just bite the bullet and refactor them with webmock next week.