tests/integrated: Do not verify buffer_id of packet_in resulting from packet_out

When a packet_in message results from a packet_out message
there is no particular relationship between the buffer_ids of
the two messages.

I noticed this when using Open vSwitch's "make ryu-check".

Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>