Remove $FreeBSD$: one-line lua tag

Remove /^--\s*\$FreeBSD\$.*$\n/

Set up the /usr/tests hierarchy.

Populate /usr/tests with the only test programs that currently live
in the tree (those in lib/libcrypt/tests/) and add all the build
machinery to accompany this change.

In particular:

- Add a WITHOUT_TESTS variable that users can define to request that
no tests be put in /usr/tests.
- Add a top-level Kyuafile for /usr/tests and a way to create similar
Kyuafiles in top-level subdirectories.
- Add a BSD.tests.dist file to define the directory layout of
/usr/tests.

Submitted by: Julio Merino jmmv google.com
Reviewed by: sjg
MFC after: 2 weeks