ipatests: refactor test_trust.py
    
    Tests in test_trust.py were organized in ten classes, one for each trust type,
    requiring ten cycles of ipaserver installation/uninstallation and the full test
    run lasted for about 5500 seconds.
    There is no need in reinstallation of ipaserver between establishing different
    types of trust.
    I moved all tests to sinle class, preserving test logic.
    
    Additional changes:
     * TestEnforcedPosixADTrust was totally removed as it was duplicate of
       TestPosixADTrust
     * code of repeated checks was moved to methods
     * A task was cretated for cleaning up DNS configuration changes made for
       establishing trust
    
    Related to https://pagure.io/freeipa/issue/7889
    
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>