tests/server-client.scm: Improve TCs

* tests/server-client.scm: Improve TCs.
  (libssh-log-printer): New procedure.
  (make-session-for-test): Set known hosts file explicitly.
  (make-server-for-test): Set DSA key.  Change log verbosity level to
  `rare'.
  (test-assert-with-log): New macro.
  ("accept, key exchange", "server-message-get", "message-get-type"):
  Start the client as a different process during the test.  Improve.
tests/server-client.scm: Fix global variables names
tests/server-client.scm (clnmsg): Improve

* tests/server-client.scm: Remove extra	global variables.
  (clnmsg): Improve.