tests: Make initial renderpass color attachment image layout UNDEFINED

This is more flexible for handling various initial image layouts and makes it
easier to add tests that avoid unexpected errors.
diff --git a/tests/vkrenderframework.cpp b/tests/vkrenderframework.cpp
index c074889..78df418 100644
--- a/tests/vkrenderframework.cpp
+++ b/tests/vkrenderframework.cpp
@@ -269,7 +269,7 @@
     att.storeOp = VK_ATTACHMENT_STORE_OP_STORE;
     att.stencilLoadOp = VK_ATTACHMENT_LOAD_OP_DONT_CARE;
     att.stencilStoreOp = VK_ATTACHMENT_STORE_OP_DONT_CARE;
-    att.initialLayout = VK_IMAGE_LAYOUT_COLOR_ATTACHMENT_OPTIMAL;
+    att.initialLayout = VK_IMAGE_LAYOUT_UNDEFINED;
     att.finalLayout = VK_IMAGE_LAYOUT_COLOR_ATTACHMENT_OPTIMAL;
 
     VkAttachmentReference ref = {};