Added method to test device manager

components/device-mgt/org.wso2.carbon.device.mgt.core/src/test/java/org/wso2/carbon/device/mgt/core/TestDeviceManager.java

@@ -121,4 +121,9 @@ public class TestDeviceManager implements DeviceManager {
         return false;
     }
 
+    @Override
+    public PlatformConfiguration getDefaultConfiguration() throws DeviceManagementException {
+        return null;
+    }
+
 }