ldb tests: Fix ldb_lmdb_size_test

ldb tests: Fix ldb_lmdb_size_test

Fix the lmdb size test which ensures that databases > 4GiB can be
written by the lmdb backend.  This test is not run as part of the normal
CI run as it exhausts the available disk on the test runners.

It was broken by changes to LDB allowing the lmdb map size to be
specified, and requiring GUID indexing by default.

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