iotests: fix expected output from gnutls

iotests: fix expected output from gnutls

Error reporting from gnutls was improved by:

  commit 57941c9c86357a6a642f9ee3279d881df4043b6d
  Author: Daniel P. Berrangé <berrange@redhat.com>
  Date:   Fri Mar 15 14:07:58 2024 +0000

    crypto: push error reporting into TLS session I/O APIs

This has the effect of changing the output from one of the NBD
tests.

Reported-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
(cherry picked from commit 48b8583698d96d6290726400789fcd51c55691b1)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>