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-concat-out.json

@@ -0,0 +1,9 @@
+{
+  "file": {
+    "nest": {
+
+    },
+    "nest.into": 2,
+    "nest.there": "too"
+  }
+}