Reenable ignored test in SqsUnboundedReaderTest

Elasticmq is used for testing SQS, unfortunately a bug prevents testing batch extension of message visibility if the request contains an invalid receipt handle:

https://github.com/softwaremill/elasticmq/issues/632

A ChangeMessageVisibilityBatchRequest that contains an entry with an invalid receipt handle fails entirely with status code 400 (InvalidParameterValue; see the SQS docs), rather than returning partial success containing a BatchResultErrorEntry with code ReceiptHandleIsInvalid for that particular entry. DeleteMessageBatchRequest behaves as expected.

Re-enable SqsUnboundedReaderTest.testExtendDeletedMessage once fixed.