Adds unit tests covering the problematic scenarios identified
around conflicting data in child owner references
Before After
package level 51% 68%
garbagecollector.go 60% 75%
graph_builder.go 50% 81%
graph.go 50% 68%
Added/improved coverage of key functions that had lacking unit test coverage:
* attemptToDeleteWorker
* attemptToDeleteItem
* processGraphChanges (added coverage of all added code)
