[test] Run test-runner system tests also when tests change.

NOTRY=true

Bug: v8:6917
Change-Id: I08a09ba6f39fc3be7a11e520d2a59ced1354f892
Reviewed-on: https://chromium-review.googlesource.com/868291Reviewed-by: Sergiy Byelozyorov <sergiyb@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50659}

tools/unittests/PRESUBMIT.py

+# 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.
+
+def CheckChangeOnCommit(input_api, output_api):
+  # TODO(machenbach): Run all unittests.
+  tests = input_api.canned_checks.GetUnitTestsInDirectory(
+      input_api, output_api, '.', whitelist=['run_tests_test.py$'])
+  return input_api.RunTests(tests)