tests, ci: fail instead of skip when IPv6 is expected

[david-yu]

Part 1 of 10 of the IPv6 series, split out of #1 and rebased on current scylladb/seastar master. Based on upstream-master (current scylladb/seastar master), so the diff here is only this PR's 2 commits.

Why

IPv6 tests skip when engine().net().supports_ipv6() is false. That probe opens a datagram channel on ::1 and swallows every exception, so anything that breaks IPv6 socket setup turns the IPv6 tests into skips and the run stays green. A green run therefore says nothing about whether an IPv6 test ran. Every later PR in this stack adds IPv6 tests, so this one goes first.

What

• tests: fail instead of skip when SEASTAR_TEST_REQUIRE_IPV6 is set (7ec32b2b1) — tests/unit/ipv6_support.hh with ipv6_available_or_skip(): skip by default, fail when SEASTAR_TEST_REQUIRE_IPV6 is set. Used by the gates master already has: ipv6_test, and the dual-stack gate socket_test gained with 731ac075f.
• ci: fail the Test job when an IPv6 test skips (5350eed5a) — sets the variable for test.yaml's Test step.

Does GHA run the IPv6 tests?

Yes. The Test job's ubuntu:26.04 container has IPv6 on loopback (supports_ipv6() binds ::1), and loopback is all the IPv6 unit tests use. This PR's Test run sets SEASTAR_TEST_REQUIRE_IPV6 and passes on every job that runs the unit tests (x86 and arm; GCC and Clang; debug, dev, release and sanitize; epoll, linux-aio and io_uring), so none of their IPv6 cases skipped. Every later PR in the stack passes the same way.

Testing

On the core-aws VM (Ubuntu 24.04, GCC 14, C++23, RelWithDebInfo, GnuTLS and OpenSSL in one tree), this PR's head builds and passes dns, ipv6, socket, rpc, httpd, httpd_openssl, tls, tls_openssl, network_interface, websocket and unix_domain with SEASTAR_TEST_REQUIRE_IPV6=1, so none of their IPv6 cases skipped.

Stack

1. tests, ci: fail instead of skip when IPv6 is expected #2 tests, ci: fail instead of skip when IPv6 is expected ← this PR
2. dns: reach nameservers over IPv6 #3 dns: reach nameservers over IPv6
3. dns: literals for any family, c-ares error text, link-local nameservers #4 dns: literals for any family, c-ares error text, link-local nameservers
4. net, http, rpc: dual-stack listeners (IPV6_V6ONLY, IPv4-mapped peers) #5 net, http, rpc: dual-stack listeners (IPV6_V6ONLY, IPv4-mapped peers)
5. net: IPv6 zones and strict literal parsing #6 net: IPv6 zones and strict literal parsing
6. tls: do not send IP literals in the SNI extension #7 tls: do not send IP literals in the SNI extension
7. tls: verify server_name on OpenSSL, opt-in #8 tls: verify server_name on OpenSSL, opt-in
8. net: ipv6_addr equality and hash, ipv4_addr(socket_address) throws instead of terminating #9 net: ipv6_addr equality and hash, ipv4_addr(socket_address) throws instead of terminating
9. net: listen on an unspecified address binds any family #10 net: listen on an unspecified address binds any family
10. http: build the Host header from an address in authority form #11 http: build the Host header from an address in authority form