Add a test for the sep helper.

js/test_cases/helpers_sep/input1.json

@@ -0,0 +1,16 @@
+{
+  "people": [
+    {
+      "name": "Alice",
+      "pet": "cat"
+    },
+    {
+      "name": "Bob",
+      "pet": "dog"
+    },
+    {
+      "name": "Chris",
+      "pet": "lizard"
+    }
+  ]
+}

js/test_cases/helpers_sep/main.dust

@@ -0,0 +1,3 @@
+{#people}
+    {name}{@sep petname="fluffy"},{pet}{petname}{/sep}
+{/people}