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/assert/assertion_format.go.tmpl

@@ -0,0 +1,4 @@
+{{.CommentFormat}}
+func {{.DocInfo.Name}}f(t TestingT, {{.ParamsFormat}}) bool {
+	return {{.DocInfo.Name}}(t, {{.ForwardedParamsFormat}})
+}