netpfil tests: IPv6 dummynet queue test

Same as the v4 test, but with IPv6.

MFC after: 2 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D31789

(cherry picked from commit 12184311c16160464a36ae05b1cd8c5e3c24fbaa)