selftest: Add a 'LABDC' testenv to mimic a preproduction test-bed

selftest: Add a 'LABDC' testenv to mimic a preproduction test-bed

One of the use-cases for the domain rename tool is to produce a lab
domain that can be used for pre-production testing of Samba.
Basically this involves taking a backup rename with --no-secrets (which
scrubs any sensitive info), and then restoring it.

This patch adds a testenv that mimics how a user would go about creating
a lab-domain. We run the same tests that we run against the restore and
rename testenvs.

Note that the rpc.echo tests for the testallowed and testdenied users
fail, because we don't backup the secrets for these users. So these
tests failing proves that the lab-DC testenv is correct.

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