Remove some ldexp subtests expecting one of two valid behaviors.

From version 4.60 of the spec:

   #
   #    "If exp is greater than +128 (single-precision) or +1024
   #    (double-precision), the value returned is undefined. If exp is less
   #    than -126 (single-precision) or -1022 (double-precision), the value
   #    returned may be flushed to zero. Additionally, splitting the value
   #    into a significand and exponent using frexp() and then reconstructing
   #    a floating-point value using ldexp() should yield the original input
   #    for zero and all finite non-denormalized values."

so, no, you can't demand a flushed-to-zero value.