Add a regression test for D17914.

[markj]

Summary

I will address the XXX comment and add more test cases, but figured I'd
put this up now.

[markj]

• Don't hardcode loopback ifname or address.
• Add a test for fragments with payload length 0.

[markj]

• Add tests for D17922
• Use random IP IDs

[markj]

Extend ip_reass__multiple_last_fragments

This test should really attempt to read the reassembled packet and verify its
contents. I'll try extending it along those lines, but I think that can be
handled in a separate commit.

[emaste]

LGTM after a somewhat cursory review.