tests: json: Add test for the deflattening function

Add a few test cases to verify that the old behaviour does not break and
that new one behaves sanely.

tests/virjsondata/deflatten-unflattened-in.json

@@ -0,0 +1,12 @@
+{
+    "file.blah": {
+        "any":"possible",
+        "combination":{
+            "of":"json",
+            "nested":{
+                "objects":"should",
+                "be":"possible"
+            }
+        }
+    }
+}