Fix the broken stubbing of 'PersistenceManager', which had gone stale as
a result of the conversion of 'Preferences' to asynchronous persistence
in commit 3f4d6e6 (2020/10/12). This caused the assertions in the
'readPersisted' continuation blocks of 3 of the 4 tests not to be
reached. Fix by stubbing the async 'persistenceManager::readPersisted'
method with a callback, instead of stubbing 'getPersisted'.
NOTE: Alternatively, we could add a testing-only 'readPersistedSync'
method to 'Preferences' for consistency, as this is how the other broken
(failing) tests resulting from 3f4d6e6 were fixed (in commit 68583d8).
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