Add a test case for nested enum, which was missed previously. (#3010)

php/tests/proto/test_no_namespace.proto

@@ -7,6 +7,7 @@ message NoNamespaceMessage {
     ZERO = 0;
   }
   NestedEnum b = 2;
+  repeated NestedEnum c = 3;
 }
 
 enum NoNamespaceEnum {