Reactivity of Aluminium Oxide
Aluminum oxide coating can relatively easily react with either strong base or acid, dissolving in them, exposing naked elemental aluminum to reaction medium. You can read more  here. 
To be concise, there are many ways to make aluminum react: you can use a base, you can use an acid, and you can scratch a sample of aluminum and coat it with mercury. Mercury would form an alloy with Al, but that wouldn't affect its reactivity towards the solution of anything, it would just make it harder for oxygen to oxidize the fresh aluminum. 
Answering you question: as far as I know, the nitrate test involves strong base, which serves as a reactant for aluminum oxide, dissolving it and letting aluminum to react further.