I would highly suggest buying a bottle of Quick Fix. Generally for pre-employment tests they don't supervise nor listen for tampering so you can get away with it rather easily, not to mention the near 100% success rate with it if used properly. Just practice with the temperature beforehand.

I'm saying this because If they hire you on the spot they might do a test right then and there; wear a pair of shorts under your pants and keep it in the pocket of the shorts just in case.

If you really don't want to sub go with the dilution sticky advice, but it isn't as reliable as QF I wouldn't think.