Remove $FreeBSD$: one line sh-pattern

Remove the $FreeBSD$ pattern added to the tests, as well as fixing the
mkimg_test.sh script to stop adding it when we rebase the tests.

Reviewed by: imp, emaste
Pull Request: https://github.com/freebsd/freebsd-src/pull/870

(cherry picked from commit 3b5c9845727a1b95098379f66223525e85e8a73f)
(cherry picked from commit 9ed4f43bc4c39dc5ed2ce1bf6003e4dcb4160bfc)
Approved-by: re (cperciva)