Testing/External: Update CMake minimum required version to match VTK

This commit also addresses warning like the following:

  CMake Warning (dev) at /path/to/VTK/CMake/vtkModuleMacros.cmake:94 (elseif):
    Policy CMP0054 is not set: Only interpret if() arguments as variables or
    keywords when unquoted.  Run "cmake --help-policy CMP0054" for policy
    details.  Use the cmake_policy command to set the policy and suppress this
    warning.

    Quoted variables like "DEPENDS" will no longer be dereferenced when the
    policy is set to NEW.  Since the policy is not set the OLD behavior will be
    used.
  Call Stack (most recent call first):
    /path/to/VTK/Rendering/Volume/module.cmake:1 (vtk_module)
    /path/to/VTK/CMake/vtkModuleMacros.cmake:431 (include)
    /path/to/VTK/CMake/vtkModuleMacros.cmake:639 (vtk_module_test)
    /path/to/VTK/CMake/vtkTestingMacros.cmake:357 (vtk_module_test_executable)
    /path/to/VTK/Rendering/Volume/Testing/Cxx/CMakeLists.txt:116 (vtk_test_cxx_executable)
  This warning is for project developers.  Use -Wno-dev to suppress it.

Testing/External/CMakeLists.txt

 # This project builds the test directories from all VTK modules as a separate
 # project outside the main VTK build tree as if they were an application.
-cmake_minimum_required(VERSION 2.8.5 FATAL_ERROR)
-
-foreach(p
-    CMP0020 # CMake 2.8.11
-    CMP0022 # CMake 2.8.12
-    CMP0025 # CMake 3.0
-    CMP0053 # CMake 3.1
-    )
-  if(POLICY ${p})
-    cmake_policy(SET ${p} NEW)
-  endif()
-endforeach()
+cmake_minimum_required(VERSION 3.3 FATAL_ERROR)
 
 project(VTKTestExternal)
 if(VTK_SOURCE_DIR OR VTK_BINARY_DIR)