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 f0d277cb0e
Follow-up to b5ea0736bb#19946
Follow-up to 904e7ecb66#19347Closes#19970
test 798 - incoming cookie header in a folded line
test 1665 - verify HTTP headers without final CRLF. Make sure all complete
headers are delivered even if the reponse is partial