Improve unit tests

[Pennycook]

Feature/behavior summary

While working on #87, it became apparent that several of the unit tests are actually end-to-end tests.

For example, instead of testing whether specific directives are parsed correctly, we currently test whether a file containing those directives results in an expected amount of code divergence.

Request attributes

• [X] Would this be a refactor of existing code?
• [ ] Does this proposal require new package dependencies?
• [ ] Would this change break backwards compatibility?

Related issues

No response

Solution description

I think we should:

• Identify the minimum set of functionality being tested by each unit test.
• Rewrite the unit tests to use only the minimum set of functionality.

If there are cases where we believe things should remain an end-to-end test, it may make sense to separate those tests out.

Additional notes

No response