cp: Split the basic_symlink test case in two.

This test case tests two different things: first, that copying a symlink
results in a file with the same contents as the target of the symlink,
rather than a second symlink, and second, that cp will refuse to copy a
file to itself, or to a link to itself, or a link to its target. Leave
the first part in basic_symlink, move the second part to a new test case
named samefile, and slightly expand both cases.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D43062