tests: force use of "NORMAL" TLS priority in test suite
    
    When generating certificates we rely on GNUTLS' built-in default setup
    for the ciphers used in the certs. We then currently run with the distro
    specific TLS priority setup which can be much stronger, to the extent
    that the certificates we generate are considered untrustworthy. We don't
    care about the quality of the ciphers we use in the test suite, so just
    force the priority to "NORMAL" which should ensure our certs are
    accepted by GNUTLS.
    
    Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>