Don't add duplicate validation features structs

* This can happen if the test itself adds VK_EXT_validation_features

util/test/demos/vk/vk_test.cpp

@@ -339,6 +339,7 @@ void VulkanGraphicsTest::Prepare(int argc, char **argv)
             {
               featuresEXT.pNext = instInfoNext;
               instInfoNext = &featuresEXT;
+              break;
             }
           }
         }