testing/prototest: add extension and required editions messages

Change-Id: I4abe9f7f8ac511845cecf4ab377b2441639dba90
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/565976
Reviewed-by: Michael Stapelberg <stapelberg@google.com>
Auto-Submit: Lasse Folger <lassefolger@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

testing/prototest/prototest_test.go

@@ -31,6 +31,7 @@ func Test(t *testing.T) {
 		(*testpb.TestRequired)(nil),
 		(*irregularpb.Message)(nil),
 		(*testpb.TestAllExtensions)(nil),
+		(*testeditionspb.TestAllExtensions)(nil),
 		(*legacypb.Legacy)(nil),
 		protoimpl.X.MessageOf((*legacy1pb.Message)(nil)).Interface(),
 	}