tests/sssh-ssshd.scm: Use test `known_hosts' file

* examples/sssh.scm.in (*option-spec*): Add `known-hosts-file' option.
  (main): Handle it.
* tests/sssh-ssshd.scm: Fix a bug in the test suite: use test
  `known_hosts' file to make sure that it has no records.
  ("ssshd, start"): Add a delay to make sure that
  ssshd started.
  ("sssh, exec"): Improve.  Do cleanup at the end of the TC.
  (*sssh-cmd*): Set `knownhosts' file explicitly.