dsdb audit: Fix timestamp tests

dsdb audit: Fix timestamp tests

Fix flapping test:
  [242(3560)/242 at 25m3s] samba4.dsdb.samdb.ldb_modules.audit_log
UNEXPECTED(failure):
  samba4.dsdb.samdb.ldb_modules.audit_log.test_operation_json_empty(none)
REASON: Exception: Exception: difftime(after, actual) >= 0
../source4/dsdb/samdb/ldb_modules/tests/test_audit_log.c:74: error:

The tests truncate the microsecond portion of the time, so the
difference could be less than 0.

Signed-off-by: Gary Lockyer <gary@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Tue Jun 26 06:09:46 CEST 2018 on sn-devel-144