CVE-2018-10919 tests: Add tests for guessing confidential attributes

CVE-2018-10919 tests: Add tests for guessing confidential attributes

Adds tests that assert that a confidential attribute cannot be guessed
by an unprivileged user through wildcard DB searches.

The tests basically consist of a set of DB searches/assertions that
get run for:
- basic searches against a confidential attribute
- confidential attributes that get overridden by giving access to the
  user via an ACE (run against a variety of ACEs)
- protecting a non-confidential attribute via an ACL that denies read-
  access (run against a variety of ACEs)
- querying confidential attributes via the dirsync controls

These tests all pass when run against a Windows Dc and all fail against
a Samba DC.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13434

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>