Clean up test suite problems

1. Fix HTML5 validation warnings (by replacing `<tt>` with `<code>`).
2. Update user JSON response processing (to match
   a16d386c3c171de5a787a403039f284f4177f75e).
3. Update user deletion status code processing to recognise 204 "No
   content" response (to match af329c1241cc555696c46b2c120ecfaa33f24230).