tests: Fix UnusedPreserveAttachment

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

Change-Id: I542b47cdf885bd9ccb96a0a113a56a849423800a
diff --git a/tests/layer_validation_tests.cpp b/tests/layer_validation_tests.cpp
index 3904e93..e72205d 100644
--- a/tests/layer_validation_tests.cpp
+++ b/tests/layer_validation_tests.cpp
@@ -14925,7 +14925,7 @@
     ASSERT_NO_FATAL_FAILURE(InitState());
     ASSERT_NO_FATAL_FAILURE(InitRenderTarget());
 
-    m_errorMonitor->SetDesiredFailureMsg(VK_DEBUG_REPORT_ERROR_BIT_EXT, "format is VK_FORMAT_UNDEFINED");
+    m_errorMonitor->SetDesiredFailureMsg(VK_DEBUG_REPORT_WARNING_BIT_EXT, "format is VK_FORMAT_UNDEFINED");
 
     VkAttachmentReference color_attach = {};
     color_attach.layout = VK_IMAGE_LAYOUT_GENERAL;