Add test cases for error details hidden behind a $ref

These indeed can be improved, as mentioned in
#817 (comment)
but it's a bit less clear exactly how yet -- rather than putting $ref
in the schema path, instead using relative_schema_path to only refer
to the schema post-$ref lookup is a bit more consistent with the current
norms, wherein what's in schema_path should be lookup-able via indexing.

But for now, they're distinguishable via .schema, which shows only the
$ref'ed schema for the second error.

jsonschema/tests/test_validators.py

@@ -1325,6 +1325,68 @@ def test_contains_none(self):
             ),
         )
 
+    def test_ref_sibling(self):
+        schema = {
+            "$defs": {"foo": {"required": ["bar"]}},
+            "properties": {
+                "aprop": {
+                    "$ref": "#/$defs/foo",
+                    "required": ["baz"]
+                    },
+                },
+            }
+
+        validator = validators.Draft202012Validator(schema)
+        e1, e2 = validator.iter_errors({"aprop": {}})
+        self.assertEqual(
+            (
+                e1.message,
+                e1.validator,
+                e1.validator_value,
+                e1.instance,
+                e1.absolute_path,
+                e1.schema,
+                e1.schema_path,
+                e1.relative_schema_path,
+                e1.json_path,
+            ),
+            (
+                "'bar' is a required property",
+                "required",
+                ["bar"],
+                {},
+                deque(["aprop"]),
+                {"required": ["bar"]},
+                deque(["properties", "aprop", "required"]),
+                deque(["properties", "aprop", "required"]),
+                "$.aprop",
+            ),
+        )
+        self.assertEqual(
+            (
+                e2.message,
+                e2.validator,
+                e2.validator_value,
+                e2.instance,
+                e2.absolute_path,
+                e2.schema,
+                e2.schema_path,
+                e2.relative_schema_path,
+                e2.json_path,
+            ),
+            (
+                "'baz' is a required property",
+                "required",
+                ["baz"],
+                {},
+                deque(["aprop"]),
+                {"$ref": "#/$defs/foo", "required": ["baz"]},
+                deque(["properties", "aprop", "required"]),
+                deque(["properties", "aprop", "required"]),
+                "$.aprop",
+            ),
+        )
+
 
 class MetaSchemaTestsMixin(object):
     # TODO: These all belong upstream