AutoTest: Code cosmetics

* use nullptr
* override
* remove old style casts
* fix most of the code model warnings

Change-Id: Ia08e846f9326ae28ca7e7d66748e25a8b817b9b1
Reviewed-by: Xing Xiong
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/autotest/qtest/qttestsettings.cpp

@@ -88,9 +88,8 @@ QString QtTestSettings::metricsTypeToOption(const MetricsType type)
         return QString("-callgrind");
     case MetricsType::Perf:
         return QString("-perf");
-    default:
-        return QString();
     }
+    return QString();
 }
 
 } // namespace Internal