selftest: Test join against DC with non-default site

selftest: Test join against DC with non-default site

Recent changes around restoring a domain that lacked
Default-First-Site-Name highlighted a problem. Normally when you join a
DC to a domain, samba-tool works out the correct site to use
automatically. However, if the join uses '--server' to select a DC, then
this doesn't work. It defaults back to Default-First-Site-Name, and the
join command fails if this site doesn't exist.

All the testenvs had Default-First-Site-Name present, so this was never
tested. Now the backupfromdc no longer has a Default-First-Site-Name
site, so running a simple join against that DC fails, highlighting the
problem.

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>