Instead, I ended up reading through several third-party guides, downloading images from random people's Google Drives and feeding magic incantations to VBoxManage - only to end up with a slow, crashy, barely functional system. Of course, the thing I originally wanted to test wasn't working at all, because... who knows.
I'm really amazed at how shitty Apple's developer experience is for people who don't own a Mac. Well, I'm going to give this script a try tomorrow - maybe that will work, somehow.