add two splom mocks (image generation is broken currently)

Author: etpinard

test/image/mocks/splom_0.json

@@ -0,0 +1,12 @@
+{
+  "data": [{
+    "type": "splom",
+    "dimensions": [{
+      "values": [1, 2, 3],
+      "label": "A"
+    }, {
+      "values": [2, 5, 6],
+      "label": "B"
+    }]
+  }]
+}