test/librbd: redo TestMockObjectMapDiffRequest.*Delta tests

Existing *Delta tests cover:

- beginning of time -> HEAD, through intermediate snap
- snap -> snap, directly
- snap -> HEAD, directly

But coverage is too weak: none of the weird OBJECT_PENDING cases and
only a single diff-iterate vs deep-copy case is tested, for example.

Coverage is missing completely for:

- beginning of time -> HEAD, directly
- beginning of time -> snap, directly
- beginning of time -> snap, through intermediate snap
- snap -> snap, through intermediate snap
- snap -> HEAD, through intermediate snap

This adds the following tests:

- FromBeginningToHead
- FromBeginningToHeadIntermediateSnap (expands FullDelta)
- FromBeginningToSnap
- FromBeginningToSnapIntermediateSnap
- FromSnapToSnap (expands IntermediateDelta)
- FromSnapToSnapIntermediateSnap
- FromSnapToHead (expands EndDelta)
- FromSnapToHeadIntermediateSnap

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