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
This can be fixed by removing the `message: 'too long'` key in app/models/group.rb, so that it uses the default. The current custom error message is not good enough anyway, it says "group_name is to long" but does not define "long".

In cases where we do use meaningful non-default messages, you can chain the matcher with an `with_message` call to make the test pass.