Add more tests for --external-ca-profile handling
    
    Add tests for remaining untested scenarios of --external-ca-profile
    handling in ipa-server-install.
    
    ipa-ca-install and ipa-cacert-manage remain untested at present.
    
    Fixes: https://pagure.io/freeipa/issue/7548
    Reviewed-By: Florence Blanc-Renaud <flo@redhat.com>