Not many. According to the test repair managers I have talked to the Zelda cart has some kind of special code that test the graphical features of the NES, but I am not 100% sure if they know what they are talking about or not. If bunnyboy has seen the code for it he might be the best person to answer the question...
With the power of CopyNES I just dumped my Zelda test cart. It is exact byte copy of standard Legend of Zelda (PRG 1). That should be expected because inside is just the normal SNROM board with NES-ZL-1 PRG chip. So if there are any hidden test codes, they went out in the production carts too!
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~~NGD
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