Cause the collection view runtime state inconsistency in its test.
Cause the collection view runtime state inconsistency in its test.

This modifies testCollectionViewDiffCalculator() to:
  - use the `TestViewController` defined in the test as the test collection
    view's `dataSource`. (previously, `dataSource` was nil)
  - call the superclass methods for `insertItems(at:)` and `deleteItems(at:)`.
    This is important because it causes UIKit to check the number of items in
    the collection before and after the update, which was the source of the
    failure.

jflinter/Dwifft,jflinter/Dwifft,jflinter/Dwifft,papertigers/Dwifft,papertigers/Dwifft