system-traffic: Fix conntrack tests

system-traffic: Fix conntrack tests

Three conntrack system-traffic tests are broken because of a recent
change 7827edcaebd8 ("Add dl_type to flow metadata for correct
interpretation of conntrack metadata"). It can be reproduced by
  $ make check-system-userspace TESTSUITEFLAGS='18 19 37'

This patch modifies the check messages to fix the breakage.

Fixes: 7827edcaebd8 ("Add dl_type to flow metadata for correct interpretation of conntrack metadata")
CC: Daniel Alvarez <dalvarez@redhat.com>
Signed-off-by: Yi-Hung Wei <yihung.wei@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>
Tested-by: William Tu <u9012063@gmail.com>