Testing a PID controller, need equation for PV
I'm not sure what you're expecting us to tell you, but:

Invent an example physical system, with a simulated behaviour, and test that.

Idea from the first hour of my Control Systems lecture back in the day:

PID controller that monitors the fill level of a bath tub, adjusts the inlet, and works against a leaky outlet would be an easy-to-simulate system.

Writing simulations requires more understanding than just writing a controller that works within, so I'm afraid: Since you didn't tell us which languages/frameworks you're working with, we'll have to assume that you'll need to write a simulator, and that means you'll have to understand the numerical and bandwidth needs of a simulator before you can tackle this.