Yes I see what you mean, seems there are a couple of options here, I'll make a start and we can 'iterate'.

I am not mad on using a temp directory because I still want to be able to keep the fake-sdcard as a true as possible mock of a real sdcard and that means pushing, cleaningup files all from the same location. It also means that the tests do not need to change at all; they just work (and I've made minimal changes in this pull you can see). Using a temp directory would mean more/different unit tests because the storage/flow would be slightly different.
I could meet you in the middle with being able to override the location with a variable in testvars? That would get your storage space out of the git repo, but keep the storage space to a common directory.