test/librbd: expand DiffIterateTest.DiffIterateDiscard

test/librbd: expand DiffIterateTest.DiffIterateDiscard

Similar to DiffIterateTest.DiffIterateDeterministic, systematically
cover the most common cases involving full-object discards.  With this
in place, issue [1] can be reproduced by any of:

    (preparatory) before snap3 is taken
    (1) beginning of time -> HEAD
    (2) snap1 -> HEAD
    (5) beginning of time -> snap3
    (6) snap1 -> snap3

Sub-object discards aren't covered here because of further issues
[2][3].

[1] https://tracker.ceph.com/issues/53897
[2] https://tracker.ceph.com/issues/63770
[3] https://tracker.ceph.com/issues/63771

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