Unit tests for Nightmare_js

[gr-im]

The unit tests for the JS part are a bit more complicated as they involve injecting a new runtime environment (e.g. nodeJS), however, I think it would be nice to have some minimum test support. There are several strategies:

• Investigation with https://github.com/mirage/alcotest/blob/main/alcotest-js.opam
• Minimal binding of an existing test framework, ie: jest that can lead to a nightmare_jest
• other suggestion

My goal is not to block the current effort on the JS runtime (and I'am very glad to see a real project as a first-step target) but I open the issue to keep that in mind.