add vpython3 to tests that run under vpython3

Change-Id: I51aece95bed6d32faf6f3f93591a7bdf459b3edc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3291901
Auto-Submit: Anthony Polito <apolito@google.com>
Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>

tests/bot_update_coverage_test.py

-#!/usr/bin/env python
+#!/usr/bin/env vpython3
 # Copyright (c) 2015 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.

tests/recipes_test.py

-#!/usr/bin/env python
+#!/usr/bin/env vpython3
 
 # Copyright (c) 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be