Clean up the CUB test CMake file.

It's grown quite messy over years of organic growth. Sort it out a bit.

Author: alliepiper

cub/cmake/CubCudaConfig.cmake

@@ -14,3 +14,16 @@ option(
   "Enable separable compilation on all targets that support it."
   OFF
 )
+
+if (CMAKE_CXX_COMPILER_ID STREQUAL "MSVC")
+  if (CUB_ENABLE_RDC_TESTS)
+    if ("${CMAKE_VERSION}" VERSION_LESS 3.27.5)
+      # https://gitlab.kitware.com/cmake/cmake/-/merge_requests/8794
+      message(
+        WARNING
+        "CMake 3.27.5 or newer is required to enable RDC tests in Visual Studio."
+      )
+      cmake_minimum_required(VERSION 3.27.5)
+    endif()
+  endif()
+endif()