netpfil tests: dummynet queue test

Test prioritisation and dummynet queues.
We need to give the pipe sufficient bandwidth for dummynet to work.
Given that we can't rely on the TCP connection failing alltogether, but
we can measure the effect of dummynet by imposing a time limit on a
larger data transfer.

If TCP is prioritised it'll get most of the pipe bandwidth and easily
manage to transfer the data in 3 seconds or less. When not prioritised
this will not succeed.

MFC after: 2 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D31788