a4091: fix 53C710 register and FIFO tests

a4091: fix 53C710 register and FIFO tests

Implement the CTEST5 ADCK/BBCK strobes, make DCMD writable, and emulate the 8-entry SCSI FIFO used by SODL, SSTAT2, and CTEST3.

Clear the SCSI FIFO on CTEST8.CLF so stale bytes are not reported after guests clear the chip FIFOs.

Fixes the ncr7xx -t register and SCSI FIFO tests.