The return value of `findOneAndDelete` can be null, causing (modern) typescript to raise `TS2531: Object is possibly 'null'`

Between 5 -> 6, the return value of `findOneAndDelete` changed. It now returns the document or null directly, see 'https://github.com/mongodb/node-mongodb-native/blob/HEAD/etc/notes/CHANGES_6.0.0.md#findoneandx-family-of-methods-will-now-return-only-the-found-document-or-null-by-default-includeresultmetadata-is-false-by-default'

By pinning the version we'll mitigate potential breaking changes in future.

The packages nyc and expect.js didn't have any active commits in many years.
jest seems like a sensible choice to switch to.

In future, the tests might be better refactored to remove the done callback pattern and use proper promises.
Another thing I couldn't easily fix is the beforeEach delay of 200ms, this makes tests unnecessarily slow.