Remove $FreeBSD$: one-line bare tag

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

Add some sample test programs.

This change adds some sample test cases to share/examples/tests/
demonstrating the basic usage of the atf and plain interfaces.

These test programs are fully-functional and are installed as part
of the test suite, which guarantees that the sample code remains
correct. However, they currently mostly serve as a placeholder for
additional examples and may be incomplete (depending on how you
look at them). I will see what else can be useful while working on
documentation.

As a bonus, the addition of these tests exercise the *.test.mk files,
one of which (plain.test.mk) was not yet in use, and also demonstrates
that it's possible to mix different kinds of test programs into the
same test suite.

Approved by: rpaulo (mentor)