Fix APU IRQ timing test

[DaveTCode]

Blarggs APU timing test (08) fails with number 3 which according to:

08.irq_timing
-------------
IRQ handler is invoked at minimum 29833 clocks after writing $00 to
$4017.

1) Passed tests
2) Too soon
3) Too late
4) Never occurred

means the IRQ happens too late. Since we pass the flag timing test it seems strange that we'd fail this one, maybe a CPU bug with when IRQ flag is checked? Ordering of events between APU & CPU?