pf tests: ensure we don't confuse different ICMP types

When creating a state for ICMP (v4 or v6) packets we only used the ID, which
means that we could confuse different ICMP types. For example, if we allowed
neighbour discovery but not ICMPv6 echo requests an ND packet could create a
state that the echo request would match.

Test that this is now fixed.

Reported by: Enrico Bassetti <e.bassetti@tudelft.nl>
MFC after: 1 day
Sponsored by: Rubicon Communications, LLC ("Netgate")