runtime: add windows callback tests
Just a copy of cgo callback tests from misc/cgo/test.
R=rsc
CC=golang-dev, hectorchu
https://golang.org/cl/5331062
diff --git a/src/pkg/runtime/export_test.go b/src/pkg/runtime/export_test.go
index 53c5fcb..c603e1b 100644
--- a/src/pkg/runtime/export_test.go
+++ b/src/pkg/runtime/export_test.go
@@ -18,6 +18,8 @@
func entersyscall()
func exitsyscall()
+func golockedOSThread() bool
var Entersyscall = entersyscall
var Exitsyscall = exitsyscall
+var LockedOSThread = golockedOSThread
