Remove $FreeBSD$: one-line sh pattern

Remove /^\s*#[#!]?\s*\$FreeBSD\$.*$\n/

split: add some tests

This should cover all of the basic functionality, as well as the recent
enhancement to use a dynamic buffer size rather than limiting patterns
and lines to MAXBSIZE.

Reviewed by: bapt
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D36324