Tests: dispose every canvas a GUI test builds; classmethod qapp fixture #189

Two test-hygiene follow-ups from the #172/#176 CI work.

1. About 40 tests/unit files build DiagramView() canvases without deleting them. A shown canvas left to the cyclic collector has its scene, items and cursors torn down in an unsafe order and segfaults in gc.collect() (tests/conftest.py _test_hygiene). It already hit test_gui_tool_cursor.py and test_routing_nodes.py, which got per-file fixes. A shared step in the hygiene fixture should cover the rest.
2. PytestRemovedIn10Warning: the module-scoped qapp fixture in tests/unit/test_start_to_end_and_units.py is an instance method.