Restore ITK4/ITK5 compatibility in Testing/itkTestMain.h threading setup

[Copilot]

A review comment flagged that itkTestMain.h had moved to an ITK5-only threading API while this repository still targets ITK4 in build configuration. This updates the test driver to compile against both ITK4 and ITK5 without changing project-wide ITK requirements.

• Compatibility fix

  • Added ITK version-gated threading includes and type selection in Testing/itkTestMain.h:
    • ITK5+: itkMultiThreaderBase.h / itk::MultiThreaderBase
    • ITK4: itkMultiThreader.h / itk::MultiThreader
  • Replaced direct itk::MultiThreader calls with a single compatibility alias.

• Behavioral scope

  • No functional change to CLI flags (--with-threads, --without-threads); only API selection at compile time was adjusted.

#include "itkVersion.h"
#if ITK_VERSION_MAJOR >= 5
#  include "itkMultiThreaderBase.h"
typedef itk::MultiThreaderBase MultiThreaderType;
#else
#  include "itkMultiThreader.h"
typedef itk::MultiThreader MultiThreaderType;
#endif

MultiThreaderType::SetGlobalDefaultNumberOfThreads(numThreads);