[test] make python recognize tools and tools/unittests as packages

The purpose of this is to simplify tools tests imports.
For example, with this commit, 'from ..v8_presubmit import TestClass' is
possible in 'tools/unittests/v8_presubmit_test.py'.

R=machenbach@chromium.org,sergiyb@chromium.org

Change-Id: Ib19a6e3af4027095c72d4c87a9d759ac85a2e1a0
Reviewed-on: https://chromium-review.googlesource.com/c/1379934Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Tamer Tas <tmrts@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58276}

tools/__init__.py

+#!/usr/bin/env python
+# Copyright 2018 the V8 project authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.

tools/unittests/__init__.py

+#!/usr/bin/env python
+# Copyright 2018 the V8 project authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.