Unit test: get constant length directly from array

We require a constant value in this case and querying the array pool
gives us a symbol.

Author: Joel Allred

jbmc/unit/solvers/strings/string_constraint_instantiation/instantiate_not_contains.cpp

@@ -105,7 +105,8 @@ refined_string_exprt make_refined_string_exprt(
 std::set<exprt>
 full_index_set(const array_string_exprt &s, array_poolt &array_pool)
 {
-  const mp_integer n = numeric_cast_v<mp_integer>(to_constant_expr(s.length()));
+  const mp_integer n =
+    numeric_cast_v<mp_integer>(to_constant_expr(array_pool.get_length(s)));
   std::set<exprt> ret;
   for(mp_integer i = 0; i < n; ++i)
     ret.insert(from_integer(i));