add tests for battery and gpuload sources

+ some tests refactoring

sources/extsysmonsources/batterysource.h

@@ -28,13 +28,13 @@ class BatterySource : public AbstractExtSysMonSource
 public:
     explicit BatterySource(QObject *parent, const QStringList args);
     virtual ~BatterySource();
+    QStringList getSources();
     QVariant data(QString source);
     QVariantMap initialData(QString source) const;
     void run();
     QStringList sources() const;
 
 private:
-    QStringList getSources();
     // configuration and values
     int m_batteriesCount = 0;
     QString m_acpiPath;