tests: py: objects.t: must use input, not output

tests: py: objects.t: must use input, not output

synproxy must never be used in output rules, doing so results in kernel
crash due to infinite recursive calls back to nf_hook_slow() for the
emitted reply packet.

Up until recently kernel lacked this validation, and now that the kernel
rejects this the test fails.  Use input to make this pass again.

A new test to ensure we reject synproxy in ouput should be added
in the near future.

Signed-off-by: Florian Westphal <fw@strlen.de>
Reviewed-by: Pablo Neira Ayuso <pablo@netfilter.org>