Simplify test running

[danfuzz]

Either:

• Figure out if the Jest setup can be made simpler, as they seem to have addressed the bug that kept ESM from working right. See [Feature]: Support package imports in jest-resolve jestjs/jest#12270.

or

• Look at the new Node-native testing framework. Is it ready for use?

[danfuzz]

Calling this one done. The test setup has been made a lot simpler. We're still using Jest and not the new(ish) one bundled with Node, but I think it's still too early to use it for at least a couple reasons.