Update style guide on unit test to match our usage

Per discussions[[1]],[[2]], this better matches what we are actually doing on projects.

[1]:https://thoughtbot.campfirenow.com/room/522129/transcript/message/1184746412#message_1184535060
[2]:https://thoughtbot.campfirenow.com/room/522129/transcript/message/1209240323#message_1209240323