Reduce width and height of zindex mocks

Author: Farkites

test/image/mocks/zindex-contour.json

@@ -15,5 +15,8 @@
         ],
         "zindex": 10
     }
-]
+],
+    "layout": {
+        "width": 400, "height": 400
+    }
 }

test/image/mocks/zindex-heatmap.json

@@ -18,5 +18,8 @@
         "ygap": 5,
         "zindex": 10
     }
-]
+],
+    "layout": {
+        "width": 400, "height": 400
+    }
 }

test/image/mocks/zindex-scatter-image.json

@@ -18,6 +18,6 @@
     }
 ],
     "layout": {
-        "width": 400, "height": 600
+        "width": 400, "height": 400
     }
 }