Do not verify-codecgen.sh on testdata

Otherwise, verify-codecgen was saying that the testdata needed to
change for some reason.

hack/verify-codecgen.sh

@@ -32,6 +32,7 @@ generated_files=($(
         -o -wholename './target' \
         -o -wholename '*/third_party/*' \
         -o -wholename '*/Godeps/*' \
+        -o -wholename '*/testdata/*' \
       \) -prune \
     \) -name '*.generated.go'))