EvalSymlinks returns error if given path or its target path don't exist.
Add a test for future improvement.

Change-Id: Ic9a4aa5eaee0fe7ac523d54d8eb3132a11b380b3
Reviewed-on: https://go-review.googlesource.com/27330Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>