test rndc showzone works for named.conf zones

test rndc showzone works for named.conf zones

Since the zone now has a reference to their respective configuration
tree, `rndc showzone` can be used for any zones (including those
defined in namedconf), without `allow-new-zones` being enabled.
Add a test for this.

The test is part of the addzone suite because showzone used to be
related to addzone, but this could be moved elsewhere in the future
if more specific tests are needed for showzone.