pf tests: extend ethernet dummynet test

Extend the existing ethernet dummynet test to also test dummynet on the
outbound direction.
This used to be a problem as traffic shaping wasn't done in the ethernet
code. It merely tagged the packet and left shaping up to the layer 3 pf
code. This works in the inbound direction, but not for outbound traffic
where we hit the L3 code first and only then the L2 code.

Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D35258