test_s3: fix test_list_multipart_upload
test_s3: fix test_list_multipart_upload

test wasn't really testing what it should have

Signed-off-by: Yehuda Sadeh <48638f9c2a62c4fd20be28d0bdad68f5cd041402@inktank.com>

smithfarm/s3-tests,andrewgaul/s3-tests,SUSE/s3-tests,piousbox/s3-tests,SUSE/s3-tests,piousbox/s3-tests,dreamhost/s3-tests,robbat2/s3-tests,Nexenta/s3-tests,Nexenta/s3-tests,ceph/s3-tests,timuralp/s3-tests,robbat2/s3-tests,timuralp/s3-tests,tobegit3hub/s3-tests,andrewgaul/s3-tests,jmunhoz/s3-tests,dreamhost/s3-tests,jmunhoz/s3-tests,Intel-bigdata/s3-tests,smithfarm/s3-tests,piousbox/s3-tests,Nexenta/s3-tests,tobegit3hub/s3-tests,Intel-bigdata/s3-tests,ceph/s3-tests,Nexenta/s3-tests