tests/client-server.scm: Improve TCs

* tests/client-server.scm: Improve TCs
  (spawn-server-thread, cancel-server-thread): Remove.
  (make-session-loop): Stop the loop when client disconnects.
  ("connect!, disconnect!", "get-protocol-version")
  ("authenticate-server, not-known", "authenticate-server, ok")
  ("get-public-key-hash", "userauth-none!, success")
  ("userauth-none!, denied", "userauth-none!, partial")
  ("userauth-password!, success", "userauth-password!, denied")
  ("userauth-password!, partial", "userauth-public-key!, success")
  ("userauth-get-list", "make-channel", "channel-open-session")
  ("channel-request-exec", "data transferring, string")
  ("data transferring, bytevector"): Start the server as a different
  process during the test.  Improve.
  (start-server/channel-test, make-session/channel-test)
  (start-server/dt-test, make-channel/dt-test): New procedures.