Tests: Fix RunCMake.CompileFeatures test NonValidTarget2 case

Mark the generated source file with the GENERATED property explicitly
since it is not marked automatically as the output of any custom
command.  Mark both the 0 and 1 variants so it works no matter the
outcome of the generator expression.

Tests/RunCMake/CompileFeatures/NonValidTarget2.cmake

@@ -4,5 +4,7 @@ set(genexvar $<COMPILE_FEATURES:cxx_final>)
 add_custom_target(copy_target
   COMMAND "${CMAKE_COMMAND}" -E copy "${CMAKE_CURRENT_SOURCE_DIR}/empty.cpp" "${CMAKE_CURRENT_BINARY_DIR}/copied_file${genexvar}.txt"
 )
+set_property(SOURCE "${CMAKE_CURRENT_BINARY_DIR}/copied_file0.cpp" PROPERTY GENERATED 1)
+set_property(SOURCE "${CMAKE_CURRENT_BINARY_DIR}/copied_file1.cpp" PROPERTY GENERATED 1)
 
 add_library(empty "${CMAKE_CURRENT_BINARY_DIR}/copied_file${genexvar}.cpp")