rgw: fix rgw test to reflect usage change

introduced by https://github.com/ceph/ceph/pull/626 because I did not --with-radosgw when compiling to check if it was ok.

otherwise src/test/cli/radosgw-admin/help.t fails when running make
check when run after a configure --with-radosgw

Signed-off-by: Loic Dachary <loic@dachary.org>