rgw: more extensive CORS testing

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: extend cors tests

Checks all sort of issues related to #6078.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Merge branch 'wip-6078' into next
Merge branch 'next'