Yikes! How long do you have....?
The simple answer is "it depends". Basic functionality might be all that's needed.
For anything being sold, EMC and safety testing are mandatory - but company policy or technical requirements might imply the need to conduct tests thare are outside the legally required minimum.
Anything going into an environment that's not benign might need temperature, shock and vibration testing.
There might be long-term reliability testing ('burn-in').
There may be testing across a large population of prototype units, or other forms of testing to see what happens when parameters such as voltage or frequency are at (or outside) the various limits in the design spec.
Do the wires break when they've been flexed a few times? How robust are the buttons? Can this product survive being wired up incorrectly by a complete, total, utter moron? What happens if it comes into contact with oil / water / whatever noxious fluid might be present in the same place as the product?