tests: don't use "dd" to allocate empty files

tests: don't use "dd" to allocate empty files

Let's use truncate -s … to create empty files (if they can be sparse)

Let's use fallocate -l … to create empty non-sparse files.

This should reduce the disk footprint of our tests a bit, given that in
most cases we won't use the allocate disk space in full, not even
remotely.