ipatests: add test for ipa-adtrust-install --add-agents
    
    Add tests checking the behavior of ipa-adtrust-install when
    adding trust agents:
    - try calling the remote method trust_enable_agent with
    a principal missing the required privilege.
    - try adding a trust agent when the remote node is stopped.
    The installer must detect that he's not able to run the remote
    commands and print a WARNING.
    - try adding a trust agent when the remote node is running.
    The WARNING must not be printed as the remote configuration is done.
    - try adding a trust agent with --enable-compat.
    The WARNING must not be printed and the Schema Compatibility plugin
    must be enabled (the entries
    cn=users/groups,cn=Schema Compatibility,cn=plugins,cn=config
    must contain a new attribute schema-compat-lookup-nsswitch
    (=user/group).
    
    Thanks to sorlov for the nightly test definitions and new test.
    
    Related: https://pagure.io/freeipa/issue/7600
    Co-authored-by: Sergey Orlov <sorlov@redhat.com>
    Signed-off-by: Florence Blanc-Renaud <flo@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>
    Reviewed-By: Sergey Orlov <sorlov@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>
    Reviewed-By: Sergey Orlov <sorlov@redhat.com>