Add tests for invalid anchors.

Closes: #301
Refs: https://json-schema.org/draft/2020-12/json-schema-core.html#name-defining-location-independe

Author: Julian

tests/draft-next/anchor.json

@@ -200,5 +200,21 @@
                 "valid": false
             }
         ]
+    },
+    {
+        "description": "invalid anchors",
+        "schema": { "$ref": "https://json-schema.org/draft/next/schema" },
+        "tests": [
+            {
+                "description": "MUST start with a letter (and not #)",
+                "data": { "$anchor" : "#foo" },
+                "valid": false
+            },
+            {
+                "description": "JSON pointers are not valid",
+                "data": { "$anchor" : "/a/b" },
+                "valid": false
+            }
+        ]
     }
 ]

tests/draft2019-09/anchor.json

@@ -200,5 +200,22 @@
                 "valid": false
             }
         ]
+    },
+    {
+        "description": "invalid anchors",
+        "comment": "Section 8.2.3",
+        "schema": { "$ref": "https://json-schema.org/draft/2019-09/schema" },
+        "tests": [
+            {
+                "description": "MUST start with a letter (and not #)",
+                "data": { "$anchor" : "#foo" },
+                "valid": false
+            },
+            {
+                "description": "JSON pointers are not valid",
+                "data": { "$anchor" : "/a/b" },
+                "valid": false
+            }
+        ]
     }
 ]

tests/draft2020-12/anchor.json

@@ -200,5 +200,22 @@
                 "valid": false
             }
         ]
+    },
+    {
+        "description": "invalid anchors",
+        "comment": "Section 8.2.2",
+        "schema": { "$ref": "https://json-schema.org/draft/2020-12/schema" },
+        "tests": [
+            {
+                "description": "MUST start with a letter (and not #)",
+                "data": { "$anchor" : "#foo" },
+                "valid": false
+            },
+            {
+                "description": "JSON pointers are not valid",
+                "data": { "$anchor" : "/a/b" },
+                "valid": false
+            }
+        ]
     }
 ]