The electric power system is intrinsically a cyber-physical system (CPS) with power flowing in the physical system and information flowing in the cyber-network. Testbeds are crucial for understanding the cyber-physical interactions and provide environments for prototyping novel applications. This study proposes a four-layer architecture for CPS testbeds with emphases on communication network emulation and networked physical components. A configurable software-defined network is employed to bridge physical components with wide-area applications for closed-loop control. In order to distribute physically coupled devices into multiple software simulations, this study proposes a data broker setup based on a distributed messaging environment to achieve low-latency data streaming. The decoupled design with data streaming allows for building testbed components as modules and running them in a distributed manner. Case studies verify the data broker setup for low-latency sensing and actuation, as well as the communication emulation setup for the desired network latency. Also illustrated is a replay attack scenario using synchrophasors in the Western Electricity Coordinating Council (WECC) 181-bus system for demonstrating the closed-loop cyber-physical simulation capability of the testbed.