test506: verify that CURLOPT_COOKIELIST takes share lock

It doesn't right now: http://curl.haxx.se/bug/view.cgi?id=1215
CURLOPT_COOKIELIST: take cookie share lock

When performing COOKIELIST operations the cookie lock needs to be taken
for the cases where the cookies are shared among multiple handles!

Verified by Benjamin Gilbert's updated test 506

Bug: http://curl.haxx.se/bug/view.cgi?id=1215
Reported-by: Benjamin Gilbert