tests: add test for server install with --no-dnssec-validation
    
    Add 2 tests related to the checks performed by ipa-server-install
    when --forwarder is specified:
    - if the forwarder is not reachable and we require dnssec validation,
    the installer must refuse to go on and exit on error.
    - if the forwarder is not reachable but --no-dnssec-validation is
    provided, the installer must continue.
    
    Related to https://pagure.io/freeipa/issue/7666
    
    Reviewed-By: Tibor Dudlak <tdudlak@redhat.com>