tests/client-server: Bugfix

* tests/client-server.scm ("data transferring, remote side abruptly closed"):
Bugfix: Don't log errors into stderr, use libssh log instead.  Remove "error"
word from the log string as surprisingly it tricks the test framework to think
that some test error has happened.

tests/client-server.scm

@@ -754,9 +754,9 @@
                     (loop (cons result data)
                           (get-bytevector-n channel (/ vect-size 2))))
                   (lambda err
-                    (format (current-error-port)
-                            "    unexpected error: ~a~%"
-                            err)
+                    (format-log/scm 'nolog
+                                    "data transferring, remote side abruptly closed"
+                                    "err: ~a" err)
                     #f))))))))))