Alas, I'm not aware of a way to test just one function from an installer.
(MSI projects have a test mode, where the UI runs but the actions that change the system do not, but I don't think there's an equivalent for pure InstallScript.)
P.S. An obvious tip that works with some types of functions: I'll occasionally test a custom function in a new, nearly empty project, pasting the code into the OnBegin event handler and calling abort or exit when the function is done.
