tests: use solara/pytest-ipywidgets to test image and contour #49

Job	Run time
build	47s
tests / Load tox environments	5s
tests / codestyle (ubuntu-latest)	22s
tests / py36-test (ubuntu-20.04)	2m 37s
tests / py38-test (ubuntu-latest)	2m 29s
tests / py37-test (macos-12)	3m 26s
tests / py310-test (macos-latest)	2m 18s
tests / py39-test (windows-latest)	6m 14s
tests / py38-notebooks (ubuntu-latest)	2m 44s
tests / py37-notebooks (macos-12)	4m 55s
tests / py39-notebooks (windows-latest)	6m 13s
visual-regression-tests	0s
	32m 10s

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