Provide full test coverage for IGListDiffKit model description strings

Summary:
Five model classes in IGListDiffKit: `IGListBatchUpdateData`, `IGListIndexPathResult`, `IGListIndexSetResult`, `IGListMoveIndex`, and `IGListMoveIndexPath` didn't have 100% test coverage because their `description` debug string methods weren't being tested.

I added a new unit test file specifically for testing description strings in IGListDiffKit wrote a basic test to verify each one.

This brings IGListDiffKit's test coverage back up to 100%.

Differential Revision: D44002058

fbshipit-source-id: ebb1bbb3c9a6d380540fc6bbedf5b8c5e5b254f1