Testing for correctness and reliability is a major challenge in the development and deployment of cloud computing platforms. Testing a cloud composed of hundreds to thousands of servers is often cost-prohibitive because of the extensive amount of hardware required. Simulation and emulation, i.e., traditional alternatives to hardware, are too abstract or too slow for testing production code in environments with many servers. We propose a testing approach that combines simulation and emulation in a cloud simulator that runs on a single processor yet enables testing of cloud management software as if the software were managing hundreds of servers and thousands of virtual machine instances. This approach alleviates a significant obstacle on the path to high-quality cloud computing systems.