tests: Only log critical errors from scapy
Since 2.4.5 scapy started issuing warnings about a few different
configurations during our tests. These are harmless, but they generate
stderr output, which upsets atf_check.
Configure scapy to only log critical errors (and thus not warnings) to
fix these tests.
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
(cherry picked from commit a26e895f3d803cc1f4ee1c2b33c61330998808b9)