I want to add WPT tests that make sure events fire or promises resolve, e.g.  https://crbug.com/774303 .

Tests with assert failure get -expected.txt with FAIL, but tests that timeout, such as if an event that should fire never fires, cause content_shell to timeout. It gets killed and no output is produced for any test in the same test file.

This is bad, if anyone adds tests that we don't pass that cause a timeout we lose test coverage for the entire file.

Should events and promises be wrapped in a t.step_timeout() or something similar or do we need to run asynchronous tests differently?