selftest: Test behaviour of DNS scavenge with an existing dNSTombstoned value

selftest: Test behaviour of DNS scavenge with an existing dNSTombstoned value

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14258

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
Autobuild-User(master): Stefan Metzmacher <metze@samba.org>
Autobuild-Date(master): Thu Feb  6 16:24:25 UTC 2020 on sn-devel-184

(cherry picked from commit c8e3c78d4f2a6f3e122fe458aa6835772290a700)

Autobuild-User(v4-12-test): Karolin Seeger <kseeger@samba.org>
Autobuild-Date(v4-12-test): Wed Feb 26 12:35:59 UTC 2020 on sn-devel-184