Try to generate baseline images in tox envs

Author: ruaridhg

tox.ini

@@ -18,4 +18,5 @@ commands =
     cp -R {toxinidir}/src/napari_matplotlib/tests/baseline {envdir}/baseline
     ls {toxinidir}/src/napari_matplotlib/tests/baseline
     python -c 'from skimage import data; data.brain()'
+    python -m pytest --mpl --mpl-generate-path=tests/baseline
     python -m pytest --mpl -v --color=yes --cov=napari_matplotlib --cov-report=xml