MFC r289172,r290254:

r289172:

Refactor the test/ Makefiles after recent changes to bsd.test.mk (r289158) and
netbsd-tests.test.mk (r289151)

- Eliminate explicit OBJTOP/SRCTOP setting
- Convert all ad hoc NetBSD test integration over to netbsd-tests.test.mk
- Remove unnecessary TESTSDIR setting
- Use SRCTOP where possible for clarity

r290254:

Remove unused variable (SRCDIR)

Add sample test programs.

This is a MFC of the following into stable/10:
- r258299 Add some sample test programs.
- r258552 Generate plain sh test programs from a source file.

As usual, "make tinderbox" clean on ref10-amd64.

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)