scripts: add an OpenSSH strict KEX interop test

Runs the OpenSSH client against the echoserver and the wolfSSH example
client against sshd, checking that each pairing negotiates the strict
KEX marker and that a session then runs over the connection.

- skip when ssh is missing or predates OpenSSH 9.6
- fall back from the ECC keys to RSA, then Ed25519, on a build without
  them, and give sshd RSA and Ed25519 host keys beside the ECDSA one
- skip the sshd half alone when sshd or ssh-keygen is missing, or sshd
  will not start unprivileged
- report the sequence number resets OpenSSH logs rather than test them,
  the wording being no contract
- run from make check through dist_noinst_SCRIPTS

Author: ejohnstown

scripts/include.am

@@ -20,6 +20,10 @@ endif
 # app wasn't built.
 dist_noinst_SCRIPTS+= scripts/sshclient.test
 
+# Not gated on a feature: the script skips itself when OpenSSH is missing
+# or predates strict KEX.
+dist_noinst_SCRIPTS+= scripts/openssh-interop.test
+
 dist_noinst_SCRIPTS+= scripts/fwd.test
 dist_noinst_SCRIPTS+= scripts/fwd-bulk.test
 endif