Without a testing infrastructure, you pretty much rely on programmers having code doing what they say it does, instead of having proof it does what they say it does.
Given human penchant to slip up and not realise problems until after they are encountered, having tests makes pretty much the difference between "science" and "hypothesis"
Given human penchant to slip up and not realise problems until after they are encountered, having tests makes pretty much the difference between "science" and "hypothesis"