ipatests: new test for trust with partially unreachable AD topology
    
    Establishing trust with partially unavailable AD hosts require usage
    of --server option. The new test checks that both commands trust-add
    and trust-fetch-domains properly use this option and also that
    trust-add correctly passes the server value when imlicitly invoking
    trust-fetch-domains.
    
    Relates to: https://pagure.io/freeipa/issue/7895.
    
    Reviewed-By: Tibor Dudlak <tdudlak@redhat.com>
    Reviewed-By: Tibor Dudlak <tdudlak@redhat.com>