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.

[tpaschalis]

Hey @bradfitz! Sorry for pinging, but I'd like to take a stab on this issue if that's okay with you!

From what I gather, we'd like move InternalBenchmark, InternalExample, and InternalTest into a new package most likely in src/internal/testing, or src/testing/internal.

Then, to avoid confusion during a couple of releases, we'd also like to alias them to their older names like type InternalBenchmark = internal.Benchmark.

I hope I'm not completely off the mark; please let me know if I've missed anything and I'll try to get a CL opened soon!