Fix FanOnOff unit test

[#73252230]

@kbenne @macumber 

FanOnOff::fanPowerRatioFunctionofSpeedRatioCurve now allows CurveCubic as an option. This alters the unit test to reflect the change.

https://github.com/NREL/OpenStudio/blob/develop/openstudiocore/resources/model/OpenStudio.idd#L13677