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/requirements.go

@@ -0,0 +1,9 @@
+package require
+
+// TestingT is an interface wrapper around *testing.T
+type TestingT interface {
+	Errorf(format string, args ...interface{})
+	FailNow()
+}
+
+//go:generate go run ../_codegen/main.go -output-package=require -template=require.go.tmpl -include-format-funcs