Add test coverage to `SubPlotWidget` for drag & drop events

[miker2]

It's great that new tests have been added for the SubPlotWidget. The tests cover programmatic adding and removing of traces and verify the state of _traces, _labels, and the pyqtgraph items, including color updates. This provides good coverage for the underlying data management logic.

To further enhance test coverage, consider adding tests that simulate drag-and-drop events to test the dragEnterEvent, dragMoveEvent, dropEvent, and _get_drop_index methods directly. pytest-qt provides tools for simulating mouse events which could be useful here.

Originally posted by @gemini-code-assist[bot] in #2 (comment)