tests/pmd.at: Fix race in "PMD - change numa node" test

tests/pmd.at: Fix race in "PMD - change numa node" test

Sometimes the test fails since dpif-netdev may process the 2 packets
in the "wrong" order.

This commit avoids the problem by printing (monitor) and verifying
any single packet instead of checking the 2 packets at the same time.

CC: Daniele Di Proietto <daniele.di.proietto@gmail.com>
Fixes: a12e2a88d672 ("test: Add more pmd tests.")
Signed-off-by: Timothy Redaelli <tredaelli@redhat.com>
Signed-off-by: Daniele Di Proietto <diproiettod@ovn.org>