ng_macfilter_test: Skip rather than fail if there is no network

This should bring the number of Jenkins failures from 4 down to 3.
Locally kyua now prints skipped: could not find a valid interface [0.115s]
when I run it in QEMU without a network device.

Reviewed By: lwhsu
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D29414