Test the fileinit and protodesc packages by verifying that passing a
FileDescriptorProto through a round trip of these packages leaves it
unchanged.
Change-Id: I6bfb894d95f1736f9908adee1ab63e9653b3f1be
Reviewed-on: https://go-review.googlesource.com/c/159762
Reviewed-by: Herbie Ong <herbie@google.com>
