Use `pytest-matplotlib` to run figure tests on generated figures

[dstansby]

[ruaridhg]

• Investigate how to remove circular reference between widget and figure
• Axes text to show up in the test figure

[dstansby]

@ruaridhg could you open these ☝️ as individual issues, and add them to the project board?

[dstansby]

Fixed by #89

[samcunliffe]

also

• get the pytest-mpl report out from tox and upload it as a build artefact in the ci for developer quality of life.