tests/client-server.scm ("connect-disconnect"): Rename TC.

* tests/client-server.scm ("connect-disconnect"): Rename TC to
  "connect!, disconnect!".
tests/client-server.scm: Add new TCs.

* tests/client-server.scm (make-server-for-test): New procedure.
  (spawn-server-thread): New macro.
  (cancel-server-thread): New procedure.
  ("userauth-get-list", "userauth-none!, success")
  ("userauth-none!, denied", "userauth-none!, partial")
  ("userauth-password!, success", "userauth-password!, denied")
  ("userauth-password!, partial"): New TCs.