_stbt.imgproc_cache unit tests: Take the minimum time of 10 executions
_stbt.imgproc_cache unit tests: Take the minimum time of 10 executions

...instead of the average. This is to fix false positives on Travis.

The minimum gives a lower bound for how fast the operation is. Higher
values will be caused by other processes competing for CPU time, which
happens especially on Travis.

LewisHaley/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,stb-tester/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester