Accuracy testing refers to any measure to determine accurate adherence to a given system's design. In emulation, accuracy tests are usually done in one of two ways.
Some homebrew is designed to look for capabilities of the original system. For many, their express goal is to make sure emulators can replicate them. Some systems can have discrepancies across hardware versions that can also be detected in software. Test ROMs may also be able to catch those as well. Some examples of test ROMs for consoles:
Demos are media productions (homebrew) that make use of a computer or console to showcase a programmer's technical prestige or knowledge, and/or the system's capabilities. They are often used to test the accuracy of an emulator.