CI task for checking xk6 compatibility

[na--]

We probably should have a simple CI task that tests k6 compatibility with https://github.com/k6io/xk6. Basically, run xk6 and bundle a simple JS and (after #1869) output extensions (e.g. write foo in a specific file) with the currently checked-out k6 code and then run a simple script, verifying the result.

Of course, we can build on that in the future and make it even more elaborate. For example, we could use these xk6 extensions for facilitating some complicated integration tests (#342).

Prompted by this discussion: #1869 (comment)