Add option to split test-file into multiple small files. (e.g. for FAT filesystem)

[mbuesch]

Older or limited filesystems such as FAT don't support huge files. When not writing to a raw device, add an option to automatically split the test files into multiple smaller files (e.g. 4 GiB - 1). Make the size of the files user selectable.

(Follow-up from #2)