Migrate tests from Mocha to Jest

[jescalada]

Although not critical, migrating the tests from Mocha (and JS) to Jest (and TS) would be great to truly complete the JS -> TS refactor, as well as help increase coverage for hard-to-test modules.

[coopernetes]

We should also consider vitest which has better ESM support for mocks. Jest's support of ES modules leaves a lot to be desired.