selftest: Don't use global dirs when parsing customdc realm

selftest: Don't use global dirs when parsing customdc realm

When creating the customdc, testparm would default to using
/usr/local/samba sub-directories for creating sockets and lock files.
Instead, pass in the tmpdir we just created as an option to the command.

Normally this didn't cause a noticeable problem, however, if we run the
command with UID-wrapper but without socket-wrapper (i.e.
USE_NAMESPACES=1), then it fails completely.

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Fri May 31 06:34:36 UTC 2019 on sn-devel-184