[Bugfix][Tests] Don't try to enable FP exceptions using feenableexcept() on OS X

OS X does not support feenableexcept(), so don't try to call it.
Enabling FP exceptions wouldn't really help anyway because that only
works for the FPU, but all floating point operations on 64-bit OS X
are done using SSE / AVX, which does not support FP exceptions.