Renode is an open-source Emulator for embedded platforms. Today, it supports x86 (Intel Quark), Cortex-A (NVIDIA Tegra), Cortex-M, SPARC (Leon), and RISC-V based platforms.
Renode can take the same firmware you are running in production, and run it against emulated cores, peripherals, and even sensors and actuators. Better yet, its extensive networking support and multi-system emulation make it a shoe in for testing systems made up of multiple devices talking together.
With Renode, you can start development before your hardware is ready, test your firmware without deploying racks of hardware, and shorten your iteration cycles by cutting out flash loading delays
This post has a good walkthrough on setting up Renode, including customizing emulated hardware and integrating with the automated testing framework.