build: unset GOPATH before tests

This is because we disallow local import for non-local packages, if
GOROOT happens to be under one of GOPATH, then some tests will fail
to build.
Fixes #3337.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5852043

src/run.bash

@@ -8,6 +8,8 @@ set -e
 eval $(go env)
 
 unset CDPATH	# in case user has it set
+unset GOPATH    # we disallow local import for non-local packages, if $GOROOT happens
+                # to be under $GOPATH, then some tests below will fail
 
 # no core files, please
 ulimit -c 0

src/run.bat

@@ -12,6 +12,10 @@ setlocal
 
 set GOBUILDFAIL=0
 
+:: we disallow local import for non-local packages, if %GOROOT% happens
+:: to be under %GOPATH%, then some tests below will fail
+set GOPATH=
+
 rem TODO avoid rebuild if possible
 
 if x%1==x--no-rebuild goto norebuild