The prototest package takes a message and exercises its implementation
of the protoreflect.Message interface in a variety of ways.
Change-Id: I4c150d1f1a6c41048f7e7f0d77efa4440ae8db0d
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/174577
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
