DevTools: add console test helpers to new test runner & migrate a console test

third_party/WebKit/LayoutTests/http/tests/inspector/devtools-js/console-clear.js

-// Copyright 2017 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.
-
-(async function() {
-  TestRunner.addResult("Tests that console is cleared upon requestClearMessages call.\n");
-
-  await TestRunner.loadPanel("console");
-  await TestRunner.loadModule("console_test_runner");
-
-  function log() {
-    // Fill console.
-    console.log("one");
-    console.log("two");
-    console.log("three");
-  }
-
-  await TestRunner.evaluateInPagePromise(`(${log.toString()})()`);
-  TestRunner.addResult("=== Before clear ===");
-  ConsoleTestRunner.dumpConsoleMessages();
-
-  Console.ConsoleView.clearConsole();
-  TestRunner.deprecatedRunAfterPendingDispatches(callback);
-  function callback() {
-    TestRunner.addResult("=== After clear ===");
-    ConsoleTestRunner.dumpConsoleMessages();
-    TestRunner.completeTest();
-  }
-})();