test: create sshd's runtime directory (Debian variant)

test: create sshd's runtime directory (Debian variant)

sshd.service on Debian uses RuntimeDirectory=sshd, without which sshd
complains:

[ 4065.834904] sshd[711]: Missing privilege separation directory: /run/sshd
[ 4065.835785] systemd[1]: mysshserver@0-127.0.0.1:4711-127.0.0.1:58232.service: Deactivated successfully.
[ 4065.836433] testsuite-46.sh[708]: kex_exchange_identification: read: Connection reset by peer
[ 4065.836433] testsuite-46.sh[708]: Connection reset by 127.0.0.1 port 4711

Resolves: #31518