When choosing the samples that are not outliers we use a half-open interval
(outlier_min <= x < outlier_max). This causes all of the samples to be
discarded when the interquartile range is 0 because none of them are less
than outlier_max. Fix the problem and make the test more consistent by
using a closed interval (outliner_min <= x <= outlier_max).
We don't need this at this deep level since callers can now
implement this limiting manually since stats.iterations is
now returned. Also, this was interfering with the -i option
to cairo-perf anyway.
