[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>

cockatrice/src/interface/widgets/visual_database_display/visual_database_display_widget.cpp

@@ -417,11 +417,6 @@ void VisualDatabaseDisplayWidget::loadNextPage()
     int start = currentPage * cardsPerPage;
     int end = qMin(start + cardsPerPage, rowCount);
 
-    // Load more cards if we are at the end of the current list and can fetch more
-    if (end >= rowCount && databaseDisplayModel->canFetchMore(QModelIndex())) {
-        databaseDisplayModel->fetchMore(QModelIndex());
-    }
-
     // Load the next page of cards and add them to the flow widget
     flowWidget->setUpdatesEnabled(false);
     loadPage(start, end);