pf tests: add more fragmentation test cases

Add more test cases for pf fragment hole counter. Also look into
final fragment of echo reply and check total length of IP packet.

MFC after: 1 week
Obtained from: OpenBSD, bluhm <bluhm@openbsd.org>, 640736615b
Sponsored by: Rubicon Communications, LLC ("Netgate")

(cherry picked from commit db100bd93036855c7688dc088b811dc7b660f51d)