Fix DisjointColumn test case

The test started failing in commit 7d7f62e, but this was not detected
due to no automatic test environment.  The commit changed the cell
numbering from "per-column (K,I,J)-ordering" to "per-plane
(I,J,K)-ordering".  Consequently, the "correct_answer" seized to be
correct.

This change restores the "correct_answer" in the ordering introduced by
commit 7d7f62e.

While here, adjust style of the DisjointColumn test case for legibility.
Remove superfluous local variable

The collections have been checked for equal size, we then only need the
size of one of them to iterate. The other boundary pointer is never used
and only generate needless compiler warnings that pollutes the output.