Tests: Fix policy version in RunCMake.ExternalProject test

Now that we have policies that need to be tested, this test cannot
just use CMAKE_VERSION for the policy level.

Tests/RunCMake/ExternalProject/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION ${CMAKE_VERSION})
+cmake_minimum_required(VERSION 3.18)
 project(${RunCMake_TEST} NONE)
 if(CMAKE_XCODE_BUILD_SYSTEM VERSION_GREATER_EQUAL 12 AND NOT RunCMake_TEST STREQUAL "Xcode-CMP0114")
   cmake_policy(SET CMP0114 NEW)