Show legend itemheight in the fill pattern mock

The mock draws five fillpattern traces with a legend. A 20px fill swatch makes each pattern visible in the legend, so the image test covers the new attribute.

Author: MannXo

test/image/mocks/scatter_fill_pattern.json

@@ -64,6 +64,7 @@
       "rows": 1,
       "columns": 2,
       "pattern": "independent"
-    }
+    },
+    "legend": { "itemheight": 20 }
   }
 }