Match a host name against subjectAltName only

OpenSSL 4.1 no longer falls back to the subject Common Name when it
checks a host name (RFC 9525), so SSLClientServerTest.TlsVerifyHostname
began failing in the Windows CI job that installs the newest OpenSSL 4.x.

Make every backend agree instead of following whichever OpenSSL is
installed. OpenSSL passes X509_CHECK_FLAG_NEVER_CHECK_SUBJECT to both the
handshake check and verify_hostname(), and Mbed TLS and wolfSSL drop their
own CN fallback. A certificate that names its host only in the CN is now
rejected on all backends, as it already was on BoringSSL.

test/gen-certs.sh

@@ -38,6 +38,11 @@ openssl req -x509 -key key.pem -sha256 -days 3650 -nodes -subj "/CN=::1" -addext
 #                 DNS:a.zz is 97.46.122.122, IP:42.46.122.122 is "*.zz".
 openssl req -x509 -key key.pem -sha256 -days 3650 -nodes -subj "/CN=san-types" -addext "subjectAltName=DNS:a.zz,IP:42.46.122.122" -out cert_san_types.pem
 
+# cert_dns_cn.pem: CN is the DNS name the tests connect to, with NO
+#                 subjectAltName. A host is named by a SAN only (RFC 9525), so
+#                 verifying it against this cert must fail.
+openssl req -x509 -key key.pem -sha256 -days 3650 -nodes -subj "/CN=localhost" -out cert_dns_cn.pem
+
 # cert_wildcard_san.pem: a leftmost wildcard next to one that is not leftmost.
 #                 "*.leftmost.example.test" matches a single label; the
 #                 wildcard in "www.*.example.test" must not be honoured.