pyldb: Add tests for ldb.Message containment testing

pyldb: Add tests for ldb.Message containment testing

These tests verify that the 'in' operator on ldb.Message is consistent
with indexing and the get() method. This means that the 'dn' element
should always be present, lookups should be case-insensitive, and use of
an invalid type should result in a TypeError.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14845

Signed-off-by: Joseph Sutton <josephsutton@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>