selftest: Add testenv for testing backup/restore

selftest: Add testenv for testing backup/restore

This adds a new testenv for testing that a DC created using the
samba-tool backup/restore can actually be started up. This actually
requires 2 new testenvs:

1. A 'backupfromdc' that solely exists to make a online backup of.
2. A 'restoredc' which takes the backup, and then uses the backup file
to do a restore, which we then start the DC based on.

The backupfromdc is just a plain vanilla AD DC. We use a separate test
env purely for this purpose, because the restoredc will use the same
domain (and so using an existing testenv would potentially interfere
with existing test cases).

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