[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>

tests/CMakeLists.txt

@@ -516,6 +516,7 @@ add_subdirectory(carddatabase)
 add_subdirectory(deck_list_model)
 add_subdirectory(deck_list_zones)
 add_subdirectory(loading_from_clipboard)
+add_subdirectory(model_contract)
 add_subdirectory(movecard_tests)
 add_subdirectory(oracle)
 add_subdirectory(settings)