test/librbd: expand TestMockObjectMapDiffRequest edge case coverage

test/librbd: expand TestMockObjectMapDiffRequest edge case coverage

For each covered edge case or error, run through the following
scenarios:

- where the edge case concerns snap_id_start
- where the edge case concerns snap_id_end
- where the edge case concerns intermediate snapshot and
  snap_id_start == 0 (diff against the beginning of time)
- where the edge case concerns intermediate snapshot and
  snap_id_start != 0 (diff from snapshot)

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
(cherry picked from commit 9931282bfd2260d654325970555cab8c617e8f14)