This commit teaches the FunctionTest to check variable scope behavior
when a subdirectory is added inside a function call. Any PARENT_SCOPE
sets in the subdirectory should affect only the function scope which
called add_subdirectory and not its parent scope.
The command "set(... PARENT_SCOPE)" should never affect the calling
scope. This improves the Function test to check that such calls in a
subdirectory scope affect the parent but not the child.