test798: force IPv4 to avoid cross-runner port aliasing

test798: force IPv4 to avoid cross-runner port aliasing

`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