osd: correct comment

SnapSet snaps are in descending order

Signed-off-by: David Zafman <david.zafman@inktank.com>
osd: Improve snapshot test

In SelfManagedSnapRollbackPP add some overlapping writes
New SelfManagedSnapOverlap creates overlapping writes

Signed-off-by: David Zafman <david.zafman@inktank.com>