ssh/session.scm (bytevector->hex-string): Fix a bug

* ssh/session.scm (bytevector->hex-string): Fix a bug: Use
  `format' from `(ice-9 format)' module.
tests/client-server.scm ("get-public-key-hash"): Check the hash

* tests/client-server.scm ("get-public-key-hash"): Check the hash.
  Check `bytevector->hex-string' return value.