os package and path/filepath package have duplicated code for
checking symlink supports in test code.
This CL tries to simplify such test code.

Change-Id: I0371488337f5e951eca699852daab9ccb16ddd62
Reviewed-on: https://go-review.googlesource.com/27331
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>