New port: devel/py-btest: Simple driver for basic unit tests

The btest is a simple framework for writing unit tests. Freely borrowing
some ideas from other packages, it's main objective is to provide an
easy-to-use, straightforward driver for a suite of shell-based tests. Each
test consists of a set of command lines that will be executed, and success
is determined based on their exit codes. btest comes with some additional
tools that can be used within such tests to compare output against a
previously established baseline.

PR: 231738
Submitted by: Shirkdog <mshirk@daemon-security.com>
Reviewed by: krion, Nathan <ndowens@yahoo.com>
Approved by: krion (mentor)
Differential Revision: https://reviews.freebsd.org/D17353