Wsieber is right. The expression is old, perhaps mediaeval, perhaps ancient, since it occurs in Latin as exceptio probat regulum or exceptio confirmat regulum. Now probat is indeed 'probes, tests', though I don't know (OED please) at what time it took on the modern sense 'proves (conclusively)': did this happen in mediaeval Latin or early modern English?

But confirmat is presumably 'confirms, bestätigt'. There is an alternative explanation for the meaning of the phrase, namely that the need to point out an exception to a rule proves or confirms that a rule exists to make an exception from. The example quoted in Chambers is that No smoking abaft the funnel implies that smoking is allowed before the funnel.