Put test commands together

Author: dstansby

tox.ini

@@ -12,5 +12,5 @@ python =
 [testenv]
 extras = testing
 commands =
-    - python -c 'from skimage import data; data.brain()'
-    - python -m pytest --mpl -v --color=yes --cov=napari_matplotlib --cov-report=xml
+    python -c 'from skimage import data; data.brain()'
+    python -m pytest --mpl -v --color=yes --cov=napari_matplotlib --cov-report=xml