Tests the signed exchange information are available when the prefetch failed.

* http://127.0.0.1:8000/loading/sxg/resources/sxg-invalid-validity-url.sxg
  failed: true
  statusCode: 200
  resourceType: other
  SignedExchangeInfo
    Error: {"message":"Validity URL must be same-origin with request URL."}
    Error: {"message":"Failed to parse SignedExchange header."}