test/fmt_v2: add simple check for input references

Specifically this test checks that the order given in the manifest is
preserved when loaded, i.e. the internal dict has the keys ordered in
the same way, independently in which way they were specified -- list
or object.

inputs/org.osbuild.ostree

@@ -36,6 +36,7 @@ SCHEMA = """
     "description": "Commit identifier",
     "oneOf": [{
       "type": "array",
+      "minItems": 1,
       "items": {
         "type": "string"
       }