Show that we can delete a user with a dangling backlink and tidy code
I was worried that if there are only backlinks to vanish and no forward links to properly delete then the control will still be marked critcial and the request will fail.
However I can't reproduce, so I'm adding the tests I wrote to be extended if we ever can.
REF: https://lists.samba.org/archive/samba/2020-February/228153.html
Signed-off-by: Andrew Bartlett abartlet@samba.org