Make it possible to easily write automated tests:

* minimally adapt pkglint(1) into a "modulino" for testability
* verify it still runs normally as a program
* create a test script with a few very simple test cases
* hook it up to 'make test'
* verify that the tests really fail if I go breaking the code under test

Meta-addresses PR pkg/46570. New BUILD_DEPENDS, but no functional
change, so no PKGREVISION bump. Approved by wiz@.

(schmonz)