test/librbd: extend DiffIterateTest.DiffIterateDeterministic{,PP}

test/librbd: extend DiffIterateTest.DiffIterateDeterministic{,PP}

Currently we are laying data only at the beginning of an object.
Extend the skeletons to write to three different offsets in the middle
and also at the end of the object.

Separately, make C and C++ API test variants slightly different in
terms of offsets being targeted to not go through exactly the same
scenario twice.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
(cherry picked from commit da69d1fdbf69cf37ba8b75ea9a230634bb25ae76)