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/doc.go

@@ -0,0 +1,28 @@
+// Package require implements the same assertions as the `assert` package but
+// stops test execution when a test fails.
+//
+// Example Usage
+//
+// The following is a complete example using require in a standard test function:
+//    import (
+//      "testing"
+//      "github.com/stretchr/testify/require"
+//    )
+//
+//    func TestSomething(t *testing.T) {
+//
+//      var a string = "Hello"
+//      var b string = "Hello"
+//
+//      require.Equal(t, a, b, "The two words should be the same.")
+//
+//    }
+//
+// Assertions
+//
+// The `require` package have same global functions as in the `assert` package,
+// but instead of returning a boolean result they call `t.FailNow()`.
+//
+// Every assertion function also takes an optional string message as the final argument,
+// allowing custom error messages to be appended to the message the assertion method outputs.
+package require