COMP: Fix unused macros related to MSVC and OS

This is to fix the -Wunused-macros generated by clang for macros related to MSVC and Microsoft.

For itkNumericTraitsTest.cxx, the _MSC_VER is added to respect the comment.

For itkTestDriver.cxx, ITK_TEST_DRIVER_PATH_SLASH / is related to non Windows machine and there is none in the code and hence it is removed. Now, for ITK_TEST_DRIVER_PATH_SLASH \\, it is not found in the entire code. This is the reason why it is removed.

Modules/Core/Common/test/itkNumericTraitsTest.cxx

@@ -17,9 +17,11 @@
  *=========================================================================*/
 
 #if !defined(ITK_LEGACY_REMOVE)
+#  ifdef _MSC_VER
 // Suppress MSVC warnings from VS2022, saying: "warning C4996: 'std::complex<T>::complex': warning STL4037: The effect
 // of instantiating the template std::complex for any type other than float, double, or long double is unspecified."
-#  define _SILENCE_NONFLOATING_COMPLEX_DEPRECATION_WARNING
+#    define _SILENCE_NONFLOATING_COMPLEX_DEPRECATION_WARNING
+#  endif
 #endif
 
 #include <iostream>

Modules/Core/TestKernel/src/itkTestDriver.cxx

@@ -84,10 +84,8 @@
 
 #if defined(_WIN32) && !defined(__CYGWIN__)
 #  define ITK_TEST_DRIVER_PATH_SEP ';'
-#  define ITK_TEST_DRIVER_PATH_SLASH '\\'
 #else
 #  define ITK_TEST_DRIVER_PATH_SEP ':'
-#  define ITK_TEST_DRIVER_PATH_SLASH '/'
 #endif