tests: Extend verbose output and return code tests

tests: Extend verbose output and return code tests

Recent changes to chain flush and zero routines incorporate proper error
propagation so trying to flush or zero a non-existent chain results in
an error. This is consistent with iptables-legacy, extend tests to make
sure it stays this way.

Also extend verbose output test to make these recent changes didn't mess
it up.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>