One who tries a thing in order to find where it fails. The tester's usefulness is measured in problems found, not in approval given.
The agent noun of test, Latin testum (an assaying vessel); a tester's usefulness has always been measured in what the trial found wrong, not in any approval it happened to give.
Whoever puts a thing under strain on purpose, looking for the point where it gives way. What counts is not approval offered but failure located — that is the entire measure of the job.