Problem: "CompletePreservationTask" code has no tests

[djjuhasz]

Describe the problem

The Enduro code to complete a preservation task in the database has no automated tests.

Solutions considered

1. Add unit tests to test the CompletePreservationTask() code

Additional context

• Related to issues #888 and #900, and also requires switching to using the persistence package to make the code testable
• Will require adding a persistence method for updating a preservation task