tests: properly clear cookies in test helper
tests: properly clear cookies in test helper

This was introduced in 2012 with cd368b5f8eed.

ether/etherpad-lite,storytouch/etherpad-lite,joequant/etherpad-lite,storytouch/etherpad-lite,ether/etherpad-lite,storytouch/etherpad-lite,joequant/etherpad-lite,joequant/etherpad-lite,joequant/etherpad-lite,ether/etherpad-lite