tests: Fix UnusedPreserveAttachment

Undefined shouldn't be an error since its a valid enumerant.

Change-Id: I542b47cdf885bd9ccb96a0a113a56a849423800a
diff --git a/layers/parameter_validation.cpp b/layers/parameter_validation.cpp
index 147bc20..67205e3 100644
--- a/layers/parameter_validation.cpp
+++ b/layers/parameter_validation.cpp
@@ -3861,7 +3861,7 @@
             std::stringstream ss;
             ss << "vkCreateRenderPass: pCreateInfo->pAttachments[" << i << "].format is VK_FORMAT_UNDEFINED. "
                << validation_error_map[VALIDATION_ERROR_00336];
-            skip |= log_msg(dev_data->report_data, VK_DEBUG_REPORT_ERROR_BIT_EXT, (VkDebugReportObjectTypeEXT)0, 0, __LINE__,
+            skip |= log_msg(dev_data->report_data, VK_DEBUG_REPORT_WARNING_BIT_EXT, (VkDebugReportObjectTypeEXT)0, 0, __LINE__,
                             VALIDATION_ERROR_00336, "IMAGE", "%s", ss.str().c_str());
         }
     }