testing: move Internal types to internal package

[bradfitz]

InternalBenchmark, InternalExample, and InternalTest predate the "internal" package mechanism.

Per a discussion here, we could probably move those three into a new internal package, and add a type alias for a few cycles before removing the testing alias.

[rsc]

This seems like a likely accept given the reactions over the past two weeks.
/cc @mpvl

Leaving open for a week for final comments.

[rsc]

No change in consensus, so accepting.