Establish unit testing pattern

[danielemery]

The project currently has a few unit tests written for pure functions but none with mocks, spys or other testing code.

A few options should be considered and then one decided upon and implemented for a few services.

Current candidates:

• jest: I'm the most familiar with this and so could hit the ground running but it always feels like it introduces a lot of overhead
• mocha, chai and sinon: These are nice and fast and simple and I appreciate the single responsibility of each. But I find the sinon documentation more geared towards old-school javascript with very few typescript & modern module examples.
• node test runner node itself now has a built in test runner, I have no idea how this compares with the others and what scope it has but it's worth a look

[danielemery]

Spent a bunch of time testing out the node test runner. It performs very well but I found compared to jest:

• The mocking interfaces are not very ergonomic
• The assertion results for arrays and deeply nested objects are not readable