test: [producers] add test_produce() functionality

Uses unittest.mock.path to patch out call to matplotlib.pyplot.savefig()

TODO: better, more-granular tests of the bits of functionality here

test/test_producers.py

@@ -70,8 +70,16 @@ class TestVisualisationProducer(unittest.TestCase):
 
     def test_produce(self):
         """Test we can call produce without error"""
-        # TODO
-        pass
+        # to do this, we need to mock out plt.savefig(path)
+        feature = mocks.MockFeature(interval=mocks.MockInterval(start=0, end=5),
+                                    source=mocks.MockSource(path="test.mp4"))
+        features = [feature]
+        visprod = producers.VisualisationProducer(features=features)
+        with unittest.mock.patch("matplotlib.pyplot.savefig") as mock_savefig:
+            visprod.produce()
+            mock_savefig.assert_called()
+
+        # TODO: more more granular test coverage of produce behaviour
 
 class TestJSONProducer(unittest.TestCase):
     def test_init_empty(self):