internal/conformance: change test package name to conformance_test
internal/conformance: change test package name to conformance_test

Since there's no conformance package aside from the test, it's cleaner
to keep the test in the conformance_test package.

Change-Id: If9ccfdfc074fa4f390956dd17d066b8613911147
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/190981
Reviewed-by: Joe Tsai <7277d7402195d5c2006c89f45718cf82bb3e527f@gmail.com>

protocolbuffers/protobuf-go,protocolbuffers/protobuf-go