The testing of cyber-physical systems requires validating device functionality for a wide range of operating conditions. The environment with which the cyber-physical device interacts, such as lungs for a medical ventilator device or a busy freeway for an autonomous vehicle, may be complex and subsequently difficult to explore all possible configurations. Computer simulations that utilize device and environment behavioral models may be used as a first stage of testing, but at some point development must occur using the real device running in real-time. We present a codesign framework for aiding cyber-physical device development where real devices or prototypes are connected to real-time models that simulate the interacting environment. Such test setups are known as digital mockups and allow for testing environment scenarios that are hard to capture with commonly-used but limited physical mockups. The framework supports model hardware/software codesign to enable models of varying speed and accuracy to be implemented within an embedded processor or as a custom coprocessor circuit on an FPGA. We describe an accompanying tool that generates code templates to reduce the time required to develop digital mockup test setups. We utilize the framework to build a digital mockup test setup for a commercial ventilator, and showcase codesign capabilities by implementing environmental models as both circuits and as instructions on a processor.