More discriminating NaN predicates for numbers.test

More discriminating NaN predicates for numbers.test

* test-suite/tests/numbers.test: (real-nan?, complex-nan?,
  imaginary-nan?): Add more discriminating NaN testing predicates
  internal to numbers.test, and convert several uses of `nan?'
  to use these instead:
   * `real-nan?' checks that its argument is real and a NaN.
   * `complex-nan?' checks that both the real and imaginary
                    parts of its argument are NaNs.
   * `imaginary-nan?' checks that its argument's real part
                      is zero and the imaginary part is a NaN.