runtests: improve XML prolog check, enable `-w` permanently, fix two tests

runtests: improve XML prolog check, enable `-w` permanently, fix two tests

To really verify the presence of the XML prolog, also in CI.

- move the prolog check from `loadtest` to `checktest`.
  (load did a soft error, silently skipping the test instead of failing)
- runtests: enable `-w` functionality permanently for all test targets,
  drop the option. It has no measurable performance impact.
- test 798, 1665: add XML prolog.
  Follow-up to f0d277cb0e3712ae4edf8f51822ffa99e9b3ec54

Follow-up to b5ea0736bbeeae717925d8eccbdb15057d8e88e9 #19946
Follow-up to 904e7ecb66519951681377758fe6b07dde28ce36 #19347

Closes #19970