sftp: add required packages to vendor

vendor/golang.org/x/crypto/curve25519/curve25519_test.go

@@ -0,0 +1,29 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package curve25519
+
+import (
+	"fmt"
+	"testing"
+)
+
+const expectedHex = "89161fde887b2b53de549af483940106ecc114d6982daa98256de23bdf77661a"
+
+func TestBaseScalarMult(t *testing.T) {
+	var a, b [32]byte
+	in := &a
+	out := &b
+	a[0] = 1
+
+	for i := 0; i < 200; i++ {
+		ScalarBaseMult(out, in)
+		in, out = out, in
+	}
+
+	result := fmt.Sprintf("%x", in[:])
+	if result != expectedHex {
+		t.Errorf("incorrect result: got %s, want %s", result, expectedHex)
+	}
+}