I'd think they'd be running on the entire system to check compatibility. What they'd also need to do is train drivers (particularly as the controls are radically different) and put some hours on the test vehicles to see how they do. Probably safer to do this on a reserved right-of-way - and that means either Spadina or St. Clair. Given it's proximity to Hillcrest, St. Clair seem logical, at least for initial training.
But at the same time, I'd think that given how different the vehicles are to the current ones, that at some point early on they'd be wanting to run some with passengers, to see if anything unexpected happens. Either from simply loading issues, or changes in the weight of the vehicle, ramp deployment, etc. Fares might be an issue (though perhaps they should test that too), though you could simply not collect fares during the testing phase. Don't know if they would do any tests with passengers or not ...