fixed bug in intdivrem test vector extraction

Created by: kevindkim723

for integer division/sqrt test vector extraction, we now handle the edge case where rd is hardwired to zero. This fixes a bug where the testbench would check against a division/remainder operation that gives a correct nonzero result, but because the assembly sets rd=zero, the testbench raises an error because the expected result is zero.

the fix is to set the testvector's answer field to Xs when encountering this edge case, in which the testbench can skip over this testvector.