This adds tests for DNS requests, and ignoring IPv6 packets on v4-only
networks.
No behavior changes. But some things are pulled out into functions.
And the mkPacket helpers previously just for tests are moved into
non-test code to be used elsewhere to reduce duplication, doing the
checksum stuff automatically.
Updates #13038
Change-Id: I4dd0b73c75b2b9567b4be3f05a2792999d83f6a3
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>