os: add test to ensure Rename returns *LinkError

os: add test to ensure Rename returns *LinkError

Updates #10061

CL 12353 updated the documentation for os.Rename to stipulate the function will
return errors of type *os.LinkError. This CL adds a test to ensure that the
implementations continue to obey this contract.

Change-Id: I41beb8c9d8356c737de251fdc6f652caab3ee636
Reviewed-on: https://go-review.googlesource.com/12329
Reviewed-by: Ian Lance Taylor <iant@golang.org>