In order to handle 'cairo-perf-trace benchmark', we need to perform the
can_run? test on the directory name as opposed to the individual trace
names. Make it so.
By ensuring that tests take longer than a couple of seconds we eliminate
systematic errors in our measurements. However, we also effectively
eliminate the synchronisation overhead. To compensate, we attempt to
estimate the overhead by reporting the difference between a single
instance and the minimum averaged instance.
The text perf-case tries to fill the region with a single text string,
but fails to detect when the current point does not advance due to an
error. This causes the perf-case to enter an infinite loop, so we break
out when the cairo_status() has been set.
