Add Exam Controller Tests

[sophiacrennan]

Problem: We need to add exam controller tests
Solution: Using the Course Controller tests as an example, write tests

Description: Since we now have a testing framework, it's time to write tests for all our controllers! You can use the testing documentation and the course controller tests as an example to write the exam controller tests.

Specs:

• write tests for each method in the exam controller
• document any issues you had that others might encounter and document the solution

If blocked, contact @VKong6019