The tests were lacking coverage for the chunk_transformer
reading multiple segment_size pieces from an input chunk.
This patch modifies test_chunk_transformer to exercise more
input chunk scenarios.
Also improve variable naming and comments in
_test_determine_chunk_destinations_prioritize
Change-Id: I4eb55ee3e87dae478828f7ccba86fec267492bd8
Related-Change: Ib9e8a6f67c2985164dd20b049c7f144f19fd1822