These tests are so slow that that they frequently time out on bots an locally, especially when other tests run in parallel using hyper-threading.
fast/selectors/matches-backtracking.html took 44.6 seconds
fast/selectors/nth-child-of-register-requirement.html took 24.3 seconds
fast/selectors/not-backtracking.html took 23.5 seconds
This one may or may not have the same root cause, I imagine that jquery does a lot of CSS selector matching.
jquery/manipulation.html took 23.0 seconds
Marked as slow in <http://trac.webkit.org/r177271>. This may not fully help, as some ports appear to detect a timeout at another layer, and report the test as "failing".