Testing - UploadedFile::fake() function does not exist

Whilst it doesn't error saying this explicitly, none of the Illuminate/Http files associated to UploadedFile include the fake() function, suggesting to me it's probably the reason why my endpoint receives a blank file (i.e. $request->file('image') is empty).

As far as I understand this function should exist in Lumen 5.4, especially having run through the composer file. Even after a composer update it still isn't there. What gives? Does anyone know?