selftest: Rework samba.dsdb locking test to samba.dsdb_lock

selftest: Rework samba.dsdb locking test to samba.dsdb_lock

This avoids running the test while samba is modifying and locking the same database,
as this can lead to a deadlock.

The deadlock is not seen in production as the LDB read lock is not held while
waiting for another process, but this test needs to do this to demonstrate
the locking safety.

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>
Autobuild-User(master): Andreas Schneider <asn@cryptomilk.org>
Autobuild-Date(master): Fri Dec  8 21:47:55 CET 2017 on sn-devel-144