Unit tests for reference.py

[rohitsanj]

• Need unit tests on a few functions in testbook/reference.py.

Command to generate above output:

$ cd testbook
$ pytest --cov=testbook --cov-report=html
$ open htmlcov/index.html # or just navigate to this directory using file explorer (or finder) and open index.html

[gravityfalls404]

I'd like to take this up