proto: make one test more general
proto: make one test more general

Tweak the "nested unknown extension" test case's resolver to not depend
on the exact message being tested. Useful for if/when we want to run
these tests on other message implementations.

Change-Id: Id1722afd8e094ddb59cb3e5440f7994c20cfa681
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/217760
Reviewed-by: Joe Tsai <ad9e03fbec35106ba742f446a098eef3d5e31d20@google.com>

protocolbuffers/protobuf-go,protocolbuffers/protobuf-go