Follow-up to 61bf638 to add the py3-specific PRESUBMIT.py test.

The change I landed in 61bf638 couldn't include the proper tests
because of crbug.com/1201918 (I think), and so I split the
test file out into this separate follow-up CL.

Bug: 1157663
Change-Id: I06697a5b76f81f0dc96ad457406a8d4d942d882c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2846635Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@google.com>

tests/PRESUBMIT.py

+# Copyright (c) 2021 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.
+
+import sys
+
+PRESUBMIT_VERSION = '2.0.0'
+
+USE_PYTHON3 = True
+
+
+def CheckUsePython3(input_api, output_api):
+  results = []
+
+  if sys.version_info.major != 3:
+    results.append(output_api.PresubmitError(
+        'Did not use Python3 for //tests/PRESUBMIT.py.'))
+
+  return results