Switching to RSpec for testing

This is the first installment of the potential switch, it illustrates the format of model testing. If anyone has any input on how I can make these tests even better, while maintaining readability, I would really appreciate the help. I purposely only tested one method so that we can focus more on the style versus what is actually being tested. I'm hoping to decide on a style before this PR is merged.  @ferrous26 @zhangsu
I don't think relying on default values specified in factories is a good idea, this makes it not immediately clear from the context why the group is valid, since the fact that a group name is given is hidden in the factory code. It might be relatively obvious in this case, but if we follow this pattern in more complicated tests the situation will probably get worse.