`test798` is the only test fetching `http://localhost:%HTTPPORT` without
`-4`, since it needs the hostname for its folded `domain=localhost`
cookie. curl tries `::1` first, but the test HTTP server is IPv4-only.
On the BSDs, IPv4 and IPv6 have separate ephemeral port namespaces, so
with every test server binding port 0, another parallel runner's
IPv6-bound server can hold the same numeric port. curl then connects to
the wrong runner's server, which cannot open its own `log/N/test798` and
closes without a response, giving exit 52 and an empty `server.input`.
This PR fixes this flake by adding `-4` matches what tests 389 and 392
already do. Linux is immune because wildcard IPv6 binds occupy the IPv4
port too. Seen in
https://github.com/curl/curl/actions/runs/29229170329/job/86749470571.
Closes#22318
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
