Update github.com/stretchr/testify to v1.2.1

cd $GOPATH/src/k8s.io/kubernetes
hack/godep-restore.sh
rm -rf Godeps
rm -rf vendor
hack/godep-save.sh
hack/update-staging-godeps.sh

vendor/github.com/stretchr/testify/assert/assertion_forward.go.tmpl

@@ -1,4 +1,5 @@
 {{.CommentWithoutT "a"}}
 func (a *Assertions) {{.DocInfo.Name}}({{.Params}}) bool {
+	if h, ok := a.t.(tHelper); ok { h.Helper() }
 	return {{.DocInfo.Name}}(a.t, {{.ForwardedParams}})
 }