[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.cpp

@@ -33,8 +33,6 @@ CardDatabaseDisplayModel::CardDatabaseDisplayModel(QObject *parent)
         invalidate();
         emit modelDirty();
     });
-
-    loadedRowCount = 0;
 }
 
 void CardDatabaseDisplayModel::setSourceModel(QAbstractItemModel *model)
@@ -45,10 +43,7 @@ void CardDatabaseDisplayModel::setSourceModel(QAbstractItemModel *model)
 
     connect(model, &QAbstractItemModel::rowsRemoved, this, [this]() { dirty(); });
 
-    connect(model, &QAbstractItemModel::modelReset, this, [this]() {
-        loadedRowCount = 0;
-        dirty();
-    });
+    connect(model, &QAbstractItemModel::modelReset, this, [this]() { dirty(); });
 }
 
 QMap<wchar_t, wchar_t> CardDatabaseDisplayModel::characterTranslation = {{L'“', L'\"'},
@@ -56,32 +51,6 @@ QMap<wchar_t, wchar_t> CardDatabaseDisplayModel::characterTranslation = {{L'“'
                                                                          {L'‘', L'\''},
                                                                          {L'’', L'\''}};
 
-bool CardDatabaseDisplayModel::canFetchMore(const QModelIndex &index) const
-{
-    return loadedRowCount < sourceModel()->rowCount(index);
-}
-
-void CardDatabaseDisplayModel::fetchMore(const QModelIndex &index)
-{
-    int remainder = sourceModel()->rowCount(index) - loadedRowCount;
-    int itemsToFetch = qMin(100, remainder);
-
-    if (itemsToFetch == 0) {
-        return;
-    }
-
-    const auto startIndex = qMin(rowCount(QModelIndex()), loadedRowCount);
-    beginInsertRows(QModelIndex(), startIndex, startIndex + itemsToFetch - 1);
-
-    loadedRowCount += itemsToFetch;
-    endInsertRows();
-}
-
-int CardDatabaseDisplayModel::rowCount(const QModelIndex &parent) const
-{
-    return QSortFilterProxyModel::rowCount(parent);
-}
-
 bool CardDatabaseDisplayModel::lessThan(const QModelIndex &left, const QModelIndex &right) const
 {
     const QString leftString = sourceModel()->data(left, CardDatabaseModel::SortRole).toString();