dummynet tests: disable for now

Disable the dummynet tests when running the ci tests. This avoids
running into the panic described in https://reviews.freebsd.org/D33064
(where an interface is removed but a dummynet queued packet still has a
pointer to it).

These tests can be re-enabled when the work in
https://reviews.freebsd.org/D33267 lands.

Sponsored by: Rubicon Communications, LLC ("Netgate")