There are a new type of tests in WPT called crash tests: http://web-platform-tests.org/writing-tests/crashtest.html
They don't have any reference and they don't use testharness.js, basically they pass if there's no a crash (sometimes they use "test-wait" similar to reftests with "reftest-wait", see bug #186045).
It'd be nice to have support to run those tests in run-webkit-tests as we're going to end up importing some of them. Right now they are treated as pixel tests somehow, as they dump the render tree (and we know that would be different in every platform, so we'd need to skip these tests for now if they're imported).
*** Bug 216921 has been marked as a duplicate of this bug. ***