add draft 2020-12 -> draft 2019-09 test

Author: gregsdennis

remotes/draft2019-09/ignore-prefixItems.json

@@ -0,0 +1,7 @@
+{
+    "$id": "http://localhost:1234/draft2019-09/ignore-prefixItems.json",
+    "$schema": "https://json-schema.org/draft/2019-09/schema",
+    "prefixItems": [
+        {"type": "string"}
+    ]
+}

tests/draft2020-12/optional/cross-draft.json

@@ -0,0 +1,18 @@
+[
+    {
+        "description": "refs to historic drafts are processed as historic drafts",
+        "schema": {
+            "$schema": "https://json-schema.org/draft/2020-12/schema",
+            "type": "array",
+            "$ref": "http://localhost:1234/draft2019-09/ignore-prefixItems.json"
+        },
+        "tests": [
+            {
+                "description": "first item not a string is valid",
+                "$comment": "if the implementation is not processing the $ref as a 2019-09 schema, this test will fail",
+                "data": [1, 2, 3],
+                "valid": true
+            }
+        ]
+    }
+]