Cloud testing has become a crucial concern due to the explosion of cloud services and requirements of reliable and high-quality service. The technologies of virtualization and infrastructures of cloud services have enabled service providers to deploy their services conveniently. However, current test models decrease the flexibility of test scenarios and test client customization. For solving these problems, this study proposes a testing service in the cloud to help the testing environment generation, deployment and execution of test scenarios. The tester can customize test scenarios involving clients, network topologies, and test scripts. In the paper, a demonstration test toolkit was implemented, and also used for testing distributed file system with various numbers of hosts, threads, and virtual network topologies.