tests/client-server.scm: Improve logging

* tests/client-server.scm: Improve logging: Redirect stderr to
  `client-server-errors.log' file.  Redirect libssh log messages to
  `client-server-libssh.log' file.  Set more verbose log level for
  sessions.  Use `test-assert-with-log' macro instead of `test-assert'.
  (libssh-log-printer): New procedure.
  (test-assert-with-log): New macro.
* tests/Makefile.am (CLEANFILES): Add log files.
* .dir-locals.el: Update.