Ensure discovery of subschemas in definitions even in newer dialects.

The JSON Schema specification, e.g. in 2020-12, uses SHOULD language
here:

    implementations SHOULD assume that "$defs" and "definitions"
    have the same behavior when [the default] meta-schema is used.

(from https://json-schema.org/draft/2020-12/json-schema-validation#name-keywords-moved-from-validat)

Author: Julian

tests/json-schema-draft-2019-09/keywords-definitions.json

@@ -0,0 +1,22 @@
+{
+  "$schema": "../../test-schema.json",
+  "registry": {
+    "http://example.com/": {
+      "definitions": {
+        "foo": {
+          "$id": "http://example.com/oh-hey-a-subschema",
+          "abc": 123
+        }
+      }
+    }
+  },
+  "tests": [
+    {
+      "ref": "http://example.com/oh-hey-a-subschema",
+      "target": {
+        "$id": "http://example.com/oh-hey-a-subschema",
+        "abc": 123
+      }
+    }
+  ]
+}

tests/json-schema-draft-2020-12/keywords-definitions.json

@@ -0,0 +1,22 @@
+{
+  "$schema": "../../test-schema.json",
+  "registry": {
+    "http://example.com/": {
+      "definitions": {
+        "foo": {
+          "$id": "http://example.com/oh-hey-a-subschema",
+          "abc": 123
+        }
+      }
+    }
+  },
+  "tests": [
+    {
+      "ref": "http://example.com/oh-hey-a-subschema",
+      "target": {
+        "$id": "http://example.com/oh-hey-a-subschema",
+        "abc": 123
+      }
+    }
+  ]
+}