xerrors: remove dependency on local path Don't assume the test is located in a path ending in /golang.org/x/xerrors. Change-Id: Ic16ba4447fc34b9f958fcae9870f331568b0fab8 Reviewed-on: https://go-review.googlesource.com/c/160697 Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>