[Tests] Use QtTest for model contracts and signal spies (#7630)

Qt6::Test was linked into every test but never used. Add a small RAII
QAbstractItemModelTester helper and apply it to the deck list model, the card
database model and display proxy, and the sets model. Add QSignalSpy coverage
for the card-added and download-settings signals, and assert explicitly that a
card row is a leaf with no child columns (the tester does not check that).

The contract checker immediately found four model bugs, fixed here:
- DeckListModel allowed children below non-zero columns.
- CardDatabaseModel reported every card as a child of every card row.
- CardDatabaseModel reported six columns for a valid (leaf) card index.
- CardDatabaseDisplayModel claimed more rows were fetchable while rowCount
  already exposed them all, so fetchMore() inserted duplicates.

Co-authored-by: Lukas Brübach <Bruebach.Lukas@bdosecurity.de>

libcockatrice_models/libcockatrice/models/database/card_database_display_model.h

@@ -42,7 +42,6 @@ private:
     QSet<QString> cardNameSet, cardTypes, cardColors;
     FilterTree *filterTree;
     FilterString *filterString;
-    int loadedRowCount;
     QTimer dirtyTimer;
     CardSearchLanguage searchLanguage;
     QString searchText;
@@ -85,9 +84,6 @@ public:
         dirtyTimer.start(20);
     }
     void clearFilterAll();
-    [[nodiscard]] int rowCount(const QModelIndex &parent = QModelIndex()) const override;
-    [[nodiscard]] bool canFetchMore(const QModelIndex &parent) const override;
-    void fetchMore(const QModelIndex &parent) override;
 signals:
     void modelDirty();