s3: Add a test for attempting to complete an unstarted multipart upload

s3: Add a test for attempting to complete an unstarted multipart upload

The AWS documentation for CompleteMultipartUpload specifies that the call will
return a 404 with the text "NoSuchUpload" to indicate that the specified
multipart upload does not exist. This could be the result of an invalid upload
ID or a multipart upload that has been aborted or completed.

Of particular significance is the requirement that this API call return a
400-series error (indicating a client-side problem) and not a 500-series error
(indicating a server-size problem).
At the time of writing, Ceph returns a 500 in this scenario which may be
handled incorrectly by client SDKs and is likely to be misinterpreted by Ceph
monitoring as an increase in errors that require operator intervention to fix.

CompleteMultipartUpload docs available at:
https://docs.aws.amazon.com/AmazonS3/latest/API/API_CompleteMultipartUpload.html

Signed-off-by: Jacques Heunis <jheunis@bloomberg.net>