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/forward_assertions.go

@@ -0,0 +1,16 @@
+package assert
+
+// Assertions provides assertion methods around the
+// TestingT interface.
+type Assertions struct {
+	t TestingT
+}
+
+// New makes a new Assertions object for the specified TestingT.
+func New(t TestingT) *Assertions {
+	return &Assertions{
+		t: t,
+	}
+}
+
+//go:generate go run ../_codegen/main.go -output-package=assert -template=assertion_forward.go.tmpl -include-format-funcs