Added a unit test for TaskPersister

Added a simple unit test which checks the methods used to save and load the set
of taskids to file. The test creates a user, adds some task ids to the
set of taskids for that user, saves it and loads it to see whether the
set loaded is the same as the one that was saved.

Change-Id: If92be8abf9a7e3ef90630a3786867f0e1ba12f3e