test: move internalChain and internalValidator to internal package

[czarcas7ic]

As suggested by @p0mvn here, we should move the internalChain and internalValidator structs to an internal package. This prevents the need for unnecessary prefixes and simplifies the chain package.

Also see https://dave.cheney.net/2019/10/06/use-internal-packages-to-reduce-your-public-api-surface

[ValarDragon]

I believe this is obsoleted by roman's e2e spike?

[p0mvn]

My e2e spike didn't cover this. I think we might want to keep this open and revisit after the spike is fully merged.

In my view, this is a low-priority issue. Could be a good first issue for someone to onboard on e2e tests though

[ValarDragon]

i think this has been obsoleted