cp: add some basic tests

There are some tests available in the NetBSD test suite, but we don't
currently pass all of those; further investigation will go into that. For
now, just add a basic test as well as a test that copies from /dev/null to a
file.

The /dev/null test confirms that the file gets created if it's empty, then
that it truncates the file if it's non-empty. This matches some usage that
was previously employed in the build and was replaced in r366042 by a
simpler shell construct.

I will also plan on coming back to expand these in due time.

MFC after: 1 week