CPU Test Mode

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Pin 30 of the 2A03 revision G can be activated to enable a special test mode for the APU. This activates registers for testing the APU at $4018-$401A at the expense of deactivating the joypad input registers at $4016-$4017.

Visual inspection of the original 2A03 revision indicates there was planned IRQ functionality from $401C-$401F, but this was left unfinished and unusable and was outright removed from later CPU revisions.