Remove $FreeBSD$: one-line sh pattern

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

patch(1): add some basic tests

Summary:
- basic: test application of patches created by diff -u at the
beginning/middle/end of file, which have differing amounts of context
before and after chunks being added
- limited_ctx: stems from PR 74127 in which a rogue line was getting added
when the patch should have been rejected. Similar behavior was
reproducible with larger contexts near the beginning/end of a file. See
r326084 for details
- file_creation: patch sourced from /dev/null should create the file
- file_nodupe: said patch sourced from /dev/null shouldn't dupe the contents
when re-applied (personal vendetta, WIP, see comment)
- file_removal: this follows from nodupe; the reverse of a patch sourced
from /dev/null is most naturally deleting the file, as is expected based
on GNU patch behavior (WIP)