I'm not sure we can. The notion of "disabling mediacore" impacts the `test` method of a preview processor which means this `generatePreviews` method we're invoking directly doesn't get invoked at all.

I'm directly invoking those methods here because (I'm pretty sure) we can't really test the full preview processing workflow because we have to be able to both upload and download files legit in the tests, which hasn't been sorted out. So the best we can do for now is invoke the pieces individually.