In-editor run test button requirements

Hi, 

I'm wondering if someone could explain a bit more about how exactly to get the little green play button on tests to show up. I find it much easier to run single tests this way instead of using guard or another test runner, but I can't quite figure out how it works.

I noticed that it disappears when I remove Test from the class name. I also noticed that it disappears when the file is named anything other than *_test or test_*. I also can't get it to show up when I use "it"/"describe" to declare tests instead of  "test". I'm a bit new to unit testing in ruby, so a lot of these might be coming from conventions that I am unfamiliar with, but I still feel it would be helpful if they were clearly documented or explained somewhere. Anyone know how it works?