Code Quality: Tidy up unit test suite

[clinkingdog]

Some of the existing unit tests aren't that useful. (For example, the converter tests are very tied to the internal implementation.) There are probably a few holes where new unit tests would add value, too. (Note, though, that this issue does not include integration tests, which are covered by another issue.)

[CheshireSwift]

On it.

[CheshireSwift]

⬜ converter.test.js
⬜ htmlGenerator.test.js
⬜ outline.test.js
☑️ utilities.test.js - Already looking okay.