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)