XML RPC test: fix test_automember_plugin
    
    With 389-DS 1.4.0.21, automember plugin also gets triggered on modify ops
    by default. This means that a member manually removed gets automatically
    re-added by the plugin.
    This behavior can be disabled by setting autoMemberProcessModifyOps=off in
    the entry cn=Auto Membership Plugin,cn=plugins,cn=config.
    
    Before 389-DS 1.4.0.21, it was possible to remove a member and the member
    did not get re-added (unless automember-rebuild was called). This former
    behavior can be forced by setting autoMemberProcessModifyOps=off.
    
    This commit fixes the test and checks the behavior when
    autoMemberProcessModifyOps=off and when autoMemberProcessModifyOps=on.
    
    Fixes: https://pagure.io/freeipa/issue/7855
    Reviewed-By: Michal Polovka <mpolovka@redhat.com>