Remove $FreeBSD$: one-line sh pattern

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

Add basic tests for echo(1)

Verify that echo(1) does not...
- ... print the trailing newline character with option '-n'.
- ... print the trailing newline character when '\c' is appended to
the end of the string.

Submitted by: shivansh
Reviewed by: asomers, ngie
MFC after: 1 month
Sponsored by: Google, Inc (GSoC 2017)
Differential Revision: D11036