AutoTest: Hide plugin class definition in the .cpp

It's not meant to be accessed from the outside.

Change-Id: I0652ec8309e678a9b28e05acab6d0f77a0b13639
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/autotest/projectsettingswidget.cpp

@@ -46,7 +46,7 @@ private:
 };
 
 ProjectTestSettingsWidget::ProjectTestSettingsWidget(Project *project)
-    : m_projectSettings(AutotestPlugin::projectSettings(project))
+    : m_projectSettings(projectSettings(project))
 {
     setGlobalSettingsId(Constants::AUTOTEST_SETTINGS_ID);