Create modulesTestState helper

[mstoykov]

Across k6 and many extensions it is common to need to recreate how k6 will execute an extension.

This in general includes creating

1. goja.Runtime
2. lib.State
3. common.InitEnv
4. modules.VU
5. an instance of a specific js module we need
6. since recently an event loop
   combine all of them and run a script.

This isn't exactly hard but there are bunch of stuff that arguably can just be extracted in a helper struct and used across k6 and extensions.

No particular design is proposed, but I will link to some such places:

• xk6-websockets
• k6/http where it isn't evne a struct but returns 5 results 🤦
• k6/ws
• k6/grpc

There are many more, I am also pretty certain there was one that went from init context evaluation to vu context ... but can't find it. (related to this comment)

[oleiade]

This would also be really useful in xk6-redis 👍🏻