If you're just simulating the temperature there's pretty much just three options:
- The thermostat will simply command the HVAC system to maximum cooling, just like with any temperature well above the setpoint.
- The thermostat will treat that value as a faulty sensor, report "error faulty sensor", and not do much in particular.
- The thermostat will crash due to an overflow of some sort as its software was never tested with inputs like that.
Personally, I'd go with, "error faulty sensor" as the most likely outcome.
(Edit: you can stimulate the temperature by setting your house on fire. Better to just simulate it)