"on your" as in "good luck on your test"

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"Per il".

So, the whole sentence is "Buona fortuna per il (tuo) test".

But "Tuo" is not necessary, because it's implied.

A little bonus:

We prefer to say "In bocca al lupo" [ "In the mouth of the wolf!" ] rather than "Buona fortuna", not to tempt fate, even if you don't believe in bad luck. :D :D :D

Typical answers to "In bocca al lupo" are "Crepi!" [ "May it die!" ] or "Crepi il lupo!" [ "May the wolf die!" ].
Another typical sentence to wish somebody good luck is "In culo alla balena!" [ "In the ass of the whale!" ], even if it's a bit rude. You can use it with close friends. ;)
Typical answers to "In culo alla balena!" are "Speriamo che non cachi!" [ "Let's hope it doesn't shit!" ] or "Fammi luce!" [ "Gimme light!" ]. :D :D :D

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wow!!! enlightening thanks for the information... I like it.. :D

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LOL, not at all! :D