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/errors.go

@@ -0,0 +1,10 @@
+package assert
+
+import (
+	"errors"
+)
+
+// AnError is an error instance useful for testing.  If the code does not care
+// about error specifics, and only needs to return the error for example, this
+// error should be used to make the test code more readable.
+var AnError = errors.New("assert.AnError general error for testing")