tests: iptables-test: Properly assert rule deletion errors

tests: iptables-test: Properly assert rule deletion errors

Capture any non-zero return code, iptables not necessarily returns 1 on
error.

A known issue with trying to delete a rule by spec is the unsupported
--set-counters option. Strip it before deleting the rule.

Fixes: c8b7aaabbe1fc ("add iptables unit test infrastructure")
Signed-off-by: Phil Sutter <phil@nwl.cc>