isbn-verifier: a check digit can only be one character (#2604)

Author: glennj

exercises/isbn-verifier/canonical-data.json

@@ -68,6 +68,15 @@
       },
       "expected": false
     },
+    {
+      "uuid": "8005b57f-f194-44ee-88d2-a77ac4142591",
+      "description": "only one check digit is allowed",
+      "property": "isValid",
+      "input": {
+        "isbn": "3-598-21508-96"
+      },
+      "expected": false
+    },
     {
       "uuid": "fdb14c99-4cf8-43c5-b06d-eb1638eff343",
       "description": "X is not substituted by the value 10",