gigasecond: add immutable test (#1743)

* Add immutable test

* Format with prettier

Author: SleeplessByte

exercises/gigasecond/canonical-data.json

@@ -59,6 +59,21 @@
         "moment": "2015-01-24T23:59:59"
       },
       "expected": "2046-10-03T01:46:39"
+    },
+    {
+      "uuid": "fcec307c-7529-49ab-b0fe-20309197618a",
+      "description": "does not mutate the input",
+      "comments": [
+        "This test case is for languages with a mutable date or datetime  ",
+        "datastructure. After running add(moment), moment should remain   ",
+        "the same."
+      ],
+      "scenarios": ["immutable"],
+      "property": "isEqual",
+      "input": {
+        "moment": "2015-01-24T23:59:59"
+      },
+      "expected": "moment === moment"
     }
   ]
 }