tests/ofproto-dpif.at: Workaround a race

This test seems to assume only the first packets in flows
counted as 'miss'.  I don't see any code ensuring that.
The test would fail if the upcall handler for the flow doesn't
run fast enough.  Workaround the problem by giving 1 second
for the miss upcall.

Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Signed-off-by: Ben Pfaff <blp@nicira.com>