ipatests: fix TestTrust::test_subordinate_suffix
    
    The test test_subordinate_suffix is failing when configuring the DNS
    for the trust, because the dnsforwardzone already exists. It was
    configured during the previous test for nonposix trust.
    
    At the end of the tests for nonposix trust, unconfigure the DNS
    and the trust before calling the subordinate_suffix test, and add
    a test cleaning up subordinate_suffix test.
    
    Fixes: https://pagure.io/freeipa/issue/8601
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>