tests/sys/netgraph/ng_macfilter_test: Fix invalid TAP output

This should allow the test to pass in Jenkins. Testing it locally now
reports "passed" instead of "invalid TAP data".

While touching this file also fix some shellcheck warnings that were
pointed out by my IDE.

Reviewed By: lwhsu, afedorov
Differential Revision: https://reviews.freebsd.org/D29054

(cherry picked from commit 65f4ff4e68afc3867781dfc8cd4faf2a8be1c74f)