Autotest: Adapt to changed settings access pattern again

Settings are created on first access now, even though this
doesn't matter much here as this happens unconditionally in
AutotestPluginPrivate ctor via m_frameworkManager.synchronizeSettings()

Change-Id: I340927cf107d7e4b2268e842d23f1a89898e8a92
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/autotest/testcodeparser.cpp

@@ -361,7 +361,7 @@ void TestCodeParser::scanForTests(const QSet<FilePath> &filePaths,
 
     using namespace Tasking;
 
-    int limit = TestSettings::instance()->scanThreadLimit();
+    int limit = testSettings().scanThreadLimit();
     if (limit == 0)
         limit = std::max(QThread::idealThreadCount() / 4, 1);
     qCDebug(LOG) << "Using" << limit << "threads for scan.";