test/librbd: add test condition for whole-object switch with fast diff enable

test/librbd: add test condition for whole-object switch with fast diff enable

This patch adds a test condition to test fast diff between two snapshots
with whole-object switch.

`TestMockObjectMapDiffRequest.EndDelta` expects diff state
`DIFF_STATE_DATA_UPDATED` which is an incorrect expectation when diff is calculated
between two snapshots.

Fixes: https://tracker.ceph.com/issues/50787
Signed-off-by: Sunny Kumar <sunkumar@redhat.com>
(cherry picked from commit 8d62c4adf74d2aee02f12bc1955490175b55bfda)