go/buildutil, cmd/guru: fix tests for symlinks in guru and gorename to account for windows

Restructure tests to account for possibility of being run on Windows
(which doesn't handle symlinks).

Change-Id: I428db26c9a1aad337d8972baa2b71468be3a2e58
Reviewed-on: https://go-review.googlesource.com/33920
Reviewed-by: Alan Donovan <adonovan@google.com>
3 files changed