ldb:tests: Add test encoding current locking behaviour during ldb_search()

ldb:tests: Add test encoding current locking behaviour during ldb_search()

Currently, a lock is not held against modifications once the final
record is returned via a callback, so modifications can be made
during the DONE callback.  This makes it hard to write modules
that interpert an ldb search result and do further processing
so will change in the future to allow the full search to be
atomic.

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Stefan Metzmacher <metze@samba.org>