tests: Tweak the backup online tests so they're generic

tests: Tweak the backup online tests so they're generic

Update backup-online tests to be more generic. We can then re-use the
common framework for other types of backups (offline, rename), and just
change what's specific to those particular cases.

This change includes asserting the restored backup's domain/realm are
correct, which we weren't doing previously but makes sense.

The new 'return samdb' is for convenience, so that child classes can
easily extend the checks we run over the restored DB.

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