regression was introduced by cce5d7152a
which incorrectly implemented the tests and fixed a wrong requirement,
CompleteMultipart should never succeed in the tests.
Currently, the test doesn't complete nor abort the multipart upload
after tesing the list operation. It caused the `cleanup()` to fail since
the object can't not be deleted which causes the bucket deletion to
fail.
This PR adds a `CompleteMultipartUpload` call to commit the object so
the cleanup can successfully delete the object and bucket.
