Test suite: don't fail DHCP check for non-IP packets.

Since we started recording the router's traffic in our PCAP
files (refs: #16148) we have started to see this check fail. Indeed,
in some of the captures I've seen the router send an ARP request,
which we don't have a case for. It is sufficient to check only IP
packets, since DHCP only uses IP packets, so we don't have to fail if
we find something else, just ignore them.

Will-fix: #16788