ipatests: fix CA less expectations
    
    The test TestServerInstall::test_ca_2_certs has a
    wrong expectation. Scenario:
    install a CA-less master with
    ipa-server-install --ca-cert-file root.pem
    where root.pem contains the CA that signed the http and ldap
    certificates + an additional (unneeded) CA cert.
    
    The test was expecting a failure, but this scenario is not
    problematic as long as the unneeded CA cert is not added.
    
    Related to https://pagure.io/freeipa/issue/6289 which has been
    closed as won't fix
    
    Reviewed-By: Fraser Tweedale <ftweedal@redhat.com>