Prepare to prevent Catch2 v3's tests from being run, and CTest targets from being added, if Catch2 is configured as a sub-project. (#2205)

This commit fixes issue that happens if the project above us uses the same variable name, thus confusing our script which see the variable scoped from the project including Catch2, rather than ours

See #2202

CMakeLists.txt

@@ -4,6 +4,8 @@ cmake_minimum_required(VERSION 3.5)
 # disable testsuite in that case
 if(NOT DEFINED PROJECT_NAME)
   set(NOT_SUBPROJECT ON)
+else()
+  set(NOT_SUBPROJECT OFF)
 endif()
 
 option(CATCH_INSTALL_DOCS "Install documentation alongside library" ON)