cmd/guru: avoid testing symlinks on OSes that do not support them

Add condition to prevent unit tests from checking cases with symlinks on
Windows and Plan 9.

Change-Id: Idc41db94a04a0daab556a26390db3f75ded7be73
Reviewed-on: https://go-review.googlesource.com/33923
Reviewed-by: Alan Donovan <adonovan@google.com>
1 file changed