Add missing tests to storage testsuite that is shared across all storage backends

• Writer:
  • blob exists, but doAppend is false
  • blob does not exists and append is true
  • Writer.Write:
    • already closed
    • already commited
    • already canceled
  • Writer.Close
    • double-close
  • Writer.Cancel
    • already closed
    • already commited
    • success
  • Writer.Commit
    • already closed
    • already commited
    • already cancelled