tests: extend workaround for gnutls private key loading failure
    
    In gnutls 3.4.3 there is a regression in the loading of private
    keys via gnutls_x509_privkey_import. We already have a workaround
    to deal with failures on older gnutls, but the error code that
    the new gnutls returns is different. Extend the workaround so that
    is checks for GNUTLS_E_REQUESTED_DATA_NOT_AVAILABLE too.
    
    See also gnutls https://bugzilla.redhat.com/show_bug.cgi?id=1250020
    
    Signed-off-by: Daniel P. Berrange <berrange@redhat.com>