Explicitly use Bash for scripts

BUG=chromium:703769
TEST=None

Change-Id: I582f61ca6eef1e22f04cbb45bc76d0d6be522925
diff --git a/bin/run_tests b/bin/run_tests
index 0f75f90..e29d3f9 100755
--- a/bin/run_tests
+++ b/bin/run_tests
@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 # Copyright 2017 The Chromium OS Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.