Fix 'fpclassify: ambiguous call' compile fail in MSVC 2022 (#2426)

Similar to #2219, we see "'fpclassify': ambiguous call" error in
test_conformance\basic\test_fpmath.cpp
due to missing constexpr at
https://github.com/KhronosGroup/OpenCL-CTS/blob/9265cbb2c274/test_conformance/basic/test_fpmath.cpp#L104
This PR fixes the issue by moving utility function isnan_fp in
testHarness.h and use it.
Note this PR doesn't modify use of isnan in many tests where only
float/double values are checked.

test_conformance/spirv_new/testBase.h

@@ -20,6 +20,7 @@
 #define _testBase_h
 
 #include "harness/compat.h"
+#include "harness/mathHelpers.h"
 #include "harness/rounding_mode.h"
 
 #include <stdio.h>

test_conformance/spirv_new/test_decorate.cpp

@@ -231,7 +231,7 @@ static inline
     f = cl_half_to_float(cl_half_from_float(f, half_rounding));
 
     To val = static_cast<To>(std::min<float>(std::max<float>(f, loVal), hiVal));
-    if (isnan(cl_half_to_float(rhs)))
+    if (isnan_fp(rhs))
     {
         val = 0;
     }