Software development for Wireless Sensor Networks (WSNs) suffers from the adverse condition that WSN software systems can usually not be tested on a system-level in their final operations environment, as WSN deployment is an expensive and time-consuming process. Several authors therefore propose to interlock application software test tightly with simulation. In this paper, we introduce an XML-based description language that allows the WSN programmer to define a common Hardware Abstraction Layer (HAL) for seamless transfer of WSN application code between WSN node target platforms and simulator-provided platforms. We show how a common network simulator can be enhanced to fully support system-level testing of WSN application code, make some comments on the resulting changes in the software development process, and finally illustrate our approach by an example.