Fixes#26874
Due to certain people (me) not being able to tell forward from backward,
the data alignment to ensure partial uploads adhere to the 256k-align
rule would potentially _reorder_ trailing buffers generated iff the
source buffers input into the sink are small enough. Which, as a fun fact,
they are in backup upload.
Change the unit test to use raw sink IO and add two unit tests (of which
the smaller size provokes the bug) that checks the same 64k buf segmented
upload backup uses.
Closesscylladb/scylladb#26938
03408b185e