trigger_script: set x bit and shebang for test

Bug: 1127205
Change-Id: I1c9aab15909f49b163563b17ef9d2888546b7081
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2642636
Commit-Queue: Wenbin Zhang <wenbinzhang@google.com>
Reviewed-by: Wenbin Zhang <wenbinzhang@google.com>
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#845682}

testing/trigger_scripts/base_test_triggerer_unittest.py

+#!/usr/bin/env vpython
 # Copyright 2020 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.

testing/trigger_scripts/perf_device_trigger_unittest.py

-#!/usr/bin/python
+#!/usr/bin/env vpython
 # Copyright 2018 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.