The subscribing witness to prove execution of private documents must be produced in all cases except the following, unless otherwise specially provided:
(1) Ancient writings which prove themselves;
(2) If from any cause the witness cannot be produced or sworn;
(3) Official bonds required by law to be approved or tested by a particular functionary;
(4) If the paper is only incidentally or collaterally material to the case;
(5) If the party executing the written instrument testifies to its execution; or
(6) If the document is self-proving or properly acknowledged.