test/librbd: make ListSnapsWholeObject actually test stuff

test/librbd: make ListSnapsWholeObject actually test stuff

Despite being added in commit 66dd53d9c4d9 ("librbd: optionally return
full object extent for any snapshot deltas") ostensibly to test the new
LIST_SNAPS_FLAG_WHOLE_OBJECT code, it surely doesn't do that because
the flag isn't even passed to MockObjectListSnapsRequest::create().

I can only guess, but it looks like snap ID 3 was intended to be
a starting point.  Otherwise, with 0 and CEPH_NOSNAP passed as snap
IDs, the overlap that is set up for the clone wouldn't affect the
computation in any way.

Use snap ID 3 as a starting point and run both with and without
LIST_SNAPS_FLAG_WHOLE_OBJECT on the same snapset to pinpoint the
difference.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>