The broom-handle test is nonsense!!! When somebody picks up a guitar for the first time, what do they do? What can they do? They can't slap down a chord or rip off a scale BUT they can run their "dominant" thumb - or whatever - over the strings!! Left-handed people will want to do that with their left-hand BUT - like-wise - a left-handed person when sat infront of keyboard/piano for the first time will also favour their left hand BUT nobody goes around asking for a left-handed piano i.e. one with the highest notes on the left. The world is set up for "orthodox" players - that is a fact! However, if a pupil has already made some progress playing "left-handed", I wouldn't - as a guitar teacher - try to make them play "orthodox".