Add a single element array test for idx_and_len.

This test is mostly because I am currently turning truthy values into single element arrays during render, so I need to make sure idx and len and not being set for non-array iteration.

js/test_cases/idx_and_len/single_element_array.json

@@ -0,0 +1,5 @@
+{
+  "things": [
+    "Alice"
+  ]
+}