Find/Replace History: write Unit Tests

[Wittmaxi]

Quoted from @HeikoKlare at #2095 (comment)

Yes, makes sense. We can have unit tests for the business logic (HistoryStore) as well as integration tests for the UI, such as simulating key up/down events to navigate through the history for both evaluating that the business logic is integrated correctly and that the UI interaction works as expected.