Issue #4826 - Use latch to replace thread-sleeping in tests

test/BaseJsTest.cpp

 /*
 * This file is part of Adblock Plus <https://adblockplus.org/>,
 * Copyright (C) 2006-2016 Eyeo GmbH
 *
 * Adblock Plus is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License version 3 as
 * published by the Free Software Foundation.
 *
 * Adblock Plus is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 11 matching lines @@
 {
   return AdblockPlus::JsEngine::New(appInfo);
 }
 
 void BaseJsTest::SetUp()
 {
   jsEngine = CreateJsEngine();
   jsEngine->SetLogSystem(std::make_shared<ThrowingLogSystem>());
   jsEngine->SetFileSystem(std::make_shared<ThrowingFileSystem>());
   jsEngine->SetWebRequest(std::make_shared<ThrowingWebRequest>());
+  engine = ToInternal(jsEngine);
 }
 
 void BaseJsTest::TearDown()
 {
-  ToInternal(jsEngine)->WaitForQuietScheduler();
+  engine->WaitForQuietScheduler();
   EXPECT_EQ(1, jsEngine.use_count());
   jsEngine.reset();
 }