ldb_mdb/tests: Tests for wrap open

ldb_mdb/tests: Tests for wrap open

Tests to ensure that the mdb_env wrapping code correctly handles
multiple ldb's point to the same physical database file.

The test_ldb_close_with_multiple_connections tests are in
ldb_mod_op_test due to the utility code it uses from
elsewhere in that test.

Signed-off-by: Gary Lockyer <gary@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>