Add tickson_boundaries_showgrid_categorical test

Author: Farkites

test/image/mocks/tickson_boundaries_showgrid_categorical.json

@@ -0,0 +1,20 @@
+{
+    "data": [
+        {
+            "type": "bar",
+            "y": [1, 2, 3, 4, 5],
+            "x": ["a", "b", "c", "d", "e"]
+        }
+    ],
+    "layout": {
+        "width": 600,
+        "height": 300,
+        "xaxis": {
+            "tickmode": "array",
+            "tickvals": [0,2,4], 
+            "ticktext": ["One", "Three", "Five"],
+            "tickson": "boundaries",
+            "showgrid": true
+        }
+    }
+}