Use vpython as the executable for generate_builbot_json_unittest.

Change-Id: I4efd4d948cf58f697714b71233cc3122c9566c78
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2319340
Auto-Submit: Garrett Beaty <gbeaty@chromium.org>
Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Reviewed-by: Rakib Hasan <rmhasan@google.com>
Commit-Queue: Stephen Martinis <martiniss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791982}

testing/buildbot/generate_buildbot_json_unittest.py

-#!/usr/bin/python
+#!/usr/bin/env vpython
 # Copyright 2017 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.