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>