Add: Acceptance test for variable expansion

Ref: https://dev.cfengine.com/issues/6866

This tests that a variable can contain the name of a list and it will
expand properly if that variable is double dereferenced.

"list" slist => { "1", "2", "3" };
"list_ref" string => "bundle.list";

reports:
  # These should be equivalent, that is both should expand identically
  "Indirect: $($(list_ref))";
  "Direct: $(bundle.list)"