goprotobuf: update extension test to new signature for testing.Main.

This is an unpleasant dependency, to put it politely.

R=dsymonds, r
CC=golang-dev
http://codereview.appspot.com/5310047

compiler/testdata/extension_test.go

@@ -201,5 +201,6 @@ func main() {
 		{"TestMessageExtension", TestMessageExtension},
 		{"TestTopLevelExtension", TestTopLevelExtension},
 	},
-		[]testing.InternalBenchmark{})
+		[]testing.InternalBenchmark{},
+		[]testing.InternalExample{})
 }