Add js_engine input test

[nizarmah]

Follow up for #37

Tests js_engine. Helps us ensure modification is performing as expected.

CI is enough

[nizarmah]

I forgot about the mangling part 🤔

That's going to make things much more difficult

[nizarmah]

Nevermind. I thought the mangle was random. Turns out it's not. But it's very likely associated with the version used.

That's good to know.

I'm not going to automate updating the "expected" files right now.

The reason behind that decision is:

• It's fairly easy to copy the expected output from the workflow logs
• It's something we need to closely review to ensure the code is actually working