Add scatter figure test

[dstansby]

Adds a figure test for scattering two images against each other.

The fix to previous issues we were having was adding the line widget = ScatterWidget(viewer). For some reason, storing the widget in a variable instead of just doing figure = ScatterWidget(viewer).figure makes the callbacks work properley... I'm not entirely sure why, and I don't propose to enter that rabbit hole.

[ruaridhg]

Works for me locally!

[ruaridhg]

I think the tests are failing because a similar issue to #129 where the order is inconsistent so generating the test figure can switch from astronaut vs astronaut_reversed to astronaut_reversed vs astronaut meaning the image comparison fails

[dstansby]

I'm guessing that's because the selected layers are stored in a set(), which doesn't guarentee the order of layers. Let me see if I can think of a fix for the tests.

[dstansby]

To give the layers a deterministic order, I've sorted them by their name - do you think that's a good soluiton?

[ruaridhg]

It works for the tests, but does that mean that it will always plot based on alphabetical order rather than order of selecting within napari? I guess it's still probably worth merging for now