tests/client-server.scm (server-thread): Rename to `*server-thread*'

* tests/client-server.scm (server-thread): Rename to
  `*server-thread*'.
  (spawn-server-thread): Update.
  (cancel-server-thread): Update.
tests/client-server.scm: Improve message handling

* tests/client-server.scm: Make sure that the received message is
  not `#f' before handling it in a server thread.
  (make-session-loop): New macro.