vendor: add github.com/stretchr/testify test dependency

github.com/stretchr/testify is a useful library for doing
assertion in tests. It makes assertions in test more less verbose and
more comfortable to read and use.

vendor/github.com/stretchr/testify/require/require.go.tmpl

@@ -0,0 +1,6 @@
+{{.Comment}}
+func {{.DocInfo.Name}}(t TestingT, {{.Params}}) {
+	if !assert.{{.DocInfo.Name}}(t, {{.ForwardedParams}}) {
+		t.FailNow()
+	}
+}