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
Then this would become
```ruby
expect(assignment).to receive(:add_group).with(nil) { grouping }
get :new, assignment_id: assignment
```
The caveat is that we have to first expect something interaction happens on the mock model and then make the interaction happens.