tests: Work out DOMSID via samdb rather than environs

tests: Work out DOMSID via samdb rather than environs

Not all testenvs have the DOMSID set as an environment variable.
However, it's easy enough to work out from querying the samdb.

This is a slight change in that we use a source4-generated loadparm
to connect to the DB (self.lp is source3-generated, presumably for
some SMB connection dependency).

This change is so we can run the ntacls_backup tests against a DC with
SMBv1 disabled (the restoredc). Note that currently the tests fail in
the smb.SMB() connection in the setUp(), so we can't run them as part
of autobuild just yet (because we can't known-fail test errors).

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13676

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