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
Similarly,
```ruby
group_name = 'g2avatar'
expect(assignment).to receive(:add_group)
  .with(group_name) { grouping }
get :new, assignment_id: assignment, new_group_name: group_name
```