Improving the docs of Running Tests

[marimeireles]

Regarding: https://github.com/webcompat/webcompat.com/blob/master/docs/tests.md

I thought it'd be a good idea to add the milestones we have to create in our repo in the doc and also make it clear that it has to be exactly the same words (at first I had some problems when I tried to run the tests because I left some blank spaces chars at the end of the words).