CYB-315 plugging more leaky state in the specs.

This is to plug more leaky state in the specs. 

```shell
    rake spec SPEC_OPTS='--seed 4921'  
```

Will show leaky state (in this case, from the engagement_index_controller_spec to the retrieve_canvas_students_spec).  Fixed it in both places.

https://jira.ets.berkeley.edu/jira/browse/CYB-315