Benchmark: fix speed tests on older devices

On most devices, when playback speed goes above 1.0, the metric used to
determine that playback is having trouble will be late frames, with
frame drop when speed is below 1. On some older devices, this behaviour changes,
which leads to an infinite loop because of the wrong metric being used
as a stop condition.
This is a workaround to get speed test to stop on older devices and get
a failure, with a score of zero.