Reporting failed test output is broken

[jdm]

https://github.com/servo/highfive/blob/master/handlers/homu_status/init.py#L28-L37 looks for the step marked as failed and grabs the stdout for it. However, since the introduction of ./mach filter-intermittents and --no-fail for test-wpt, highfive is now grabbing the output of filter-intermittent's stdout, which is always empty.