Add: Acceptance test for variable expansion

Ref: https://dev.cfengine.com/issues/6866

When an array value is the name of a list the list fails to expand when the
array is double expanded.

For example:
"list" slist => { "a", "b", "c" };
"array[key]" string => "list";

$($(array[key])) fails to expand to a,b,c and instead expands to
$(list).