k6-chaijs: Add ability to pass custom tags to describe/expect

[thelgeland-andovercos]

With Checks you can pass in custom tags that get used in thresholds and can be used to fail tests when these checks fail.

k6-chaijs does not appear to support this currently, but it would be very useful if it did.

Per Tom Miseur:

Regarding tags: behind-the-scenes, describe calls group, meaning metrics that were emitted within the describe block will feature the group tag. Admittedly, though, that's not quite the same as passing arbitrary tags to the check function itself. I think that might be a good issue to submit against the repo.