TaskTree: Prepare for de-utils-ization - part 2

Move TaskTree into Tasking namespace.
Move Tasking namespace out of Utils namespace.

Change-Id: Ib4c1d7f54f1808517e54768dfa27209c33517b61
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: hjk <hjk@qt.io>

src/plugins/autotest/testrunner.cpp

@@ -46,6 +46,7 @@
 
 using namespace Core;
 using namespace ProjectExplorer;
+using namespace Tasking;
 using namespace Utils;
 
 namespace Autotest {
@@ -346,7 +347,6 @@ void TestRunner::runTestsHelper()
         std::unique_ptr<TestOutputReader> m_outputReader;
     };
 
-    using namespace Tasking;
     QList<TaskItem> tasks{optional};
 
     for (ITestConfiguration *config : m_selectedTests) {