rgw: fix rgw test to reflect usage change

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>
(cherry picked from commit e50343e4423e20130035c860ba47a0edea876f7c)