xs:list + seq: Add tests for nested xs:list into element sequence

failures (8):
    seq::fixed_name::fixed_size::xs_list::many
    seq::fixed_name::fixed_size::xs_list::one
    seq::fixed_name::variable_size::xs_list::many
    seq::fixed_name::variable_size::xs_list::one
    seq::variable_name::fixed_size::xs_list::many
    seq::variable_name::fixed_size::xs_list::one
    seq::variable_name::variable_size::xs_list::many
    seq::variable_name::variable_size::xs_list::one