When I debug DumpRenderTree using lldb, I typically run it from the root of the working copy, so passing "LayoutTests/inspector/test-name.html" works fine. But if I pass the same as an argument to DumpRenderTree or WebKitTestRunner from a custom Xcode scheme, I get an obtuse sandbox extension error. Could not create a sandbox extension for '/Users/bburg/repos/webkit/OpenSource/WebKitBuild/Vanilla/Debug/LayoutTests/inspector/test-harness-trivially-works.html' It seems that the CWD when running from Xcode is the BUILT_PRODUCTS_DIR, so its trying to open a file that doesn't exist with that base directory. That's fine, but it would save a lot of confusion if the error message said the file did not exist.
Same thing happens when opening a local file that references a non-existent subresource. Yes, it's not right to log a warning any time a non-existent local file is referenced.