Add binary conformance tests for message set encoding

1. straight forward message set group encoding
 2. group encoding but with the fields out of order
 3. message set encoded as a typical extension submessage
 4. A variant of the former, but I "abuse" a oneof to detect when implementations put the message set fields into the unknown field set.

Go seems to be the only implementation that fails (3) as it will drop these fields entirely. A couple of implementations fail (4).

PiperOrigin-RevId: 723946259