Fix TLS test suites with gnutls 3.6.0

With gnutls 3.6.0, SHA1 is no longer accepted for certificate
signatures. We must usw SHA256 instead.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

tests/virnettlshelpers.c

@@ -384,7 +384,7 @@ testTLSGenerateCert(struct testTLSCertReq *req,
      * If no 'ca' is set then we are self signing
      * the cert. This is done for the root CA certs
      */
-    if ((err = gnutls_x509_crt_sign(crt, ca ? ca : crt, privkey)) < 0) {
+    if ((err = gnutls_x509_crt_sign2(crt, ca ? ca : crt, privkey, GNUTLS_DIG_SHA256, 0)) < 0) {
         VIR_WARN("Failed to sign certificate %s", gnutls_strerror(err));
         abort();
     }