test_s3: fix test_list_multipart_upload()

Test was calling _multipart_upload() with extra useless param, this
broke when we added a new param to the function.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
(cherry picked from commit f4de9f7b42e2fd2e3feaa0860367f4f442777b9f)