misc/cgo/test: add missing import

Fixes windows build.

R=golang-dev
CC=golang-dev
https://golang.org/cl/9429043

misc/cgo/test/issue5337w.go

@@ -6,4 +6,6 @@
 
 package cgotest
 
+import "testing"
+
 func test5337(t *testing.T) {}