AutoTest: Access plugin singleton through static functions

It's the more common pattern nowadays, and cheaper.

Change-Id: If6217b2a820fbfa6f088fd9349225f5f8488f593
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/autotest/testrunner.cpp

@@ -393,7 +393,7 @@ void TestRunner::runTests()
     }
 
     QFuture<TestResultPtr> future = Utils::runAsync(&performTestRun, m_selectedTests,
-                                                    *AutotestPlugin::instance()->settings());
+                                                    *AutotestPlugin::settings());
     m_futureWatcher.setFuture(future);
     Core::ProgressManager::addTask(future, tr("Running Tests"), Autotest::Constants::TASK_INDEX);
 }