It would have been used on a lot check before soldering the 2A03 to the mainboard, so whatever form the test program came in, it was probably not a conventional Famicom cartridge.
Unlike the field test cartridges, where a service tech is paying attention (and can say "oh, the sound works/doesn't"), this TST interface allows the CPU itself to say "yes, everything works".
Revising the mask is a fairly expensive process (in terms of time), and while they could have changed the wire bonder to tie the TST land to ground internally in the chip and left the pin truly NC, that's not really any easier than soldering the pin low.
2a0x APU "TEST" features.
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