Check that single layer widget plots change w/ layer change #151
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This generalises the test I added in #146 to make it easy to test any widget that takes a single layer as input. Although we only have two widgets at the moment, any new widgets just need to be added to
@pytest.mark.parametrize("widget_cls", [HistogramWidget, SliceWidget])in the future and they'll be automatically tested.More work towards #7.