Two Chinese supertankers carrying a combined 4 million barrels of Saudi Arabian oil are exiting the Red Sea via the Bab el-Mandeb strait on Thursday, shipping data showed, even as a Saudi vessel came under attack in the region.The ships' attempt to transit the strait will clearly test how tightly the Houthis will impose the naval blockade on Saudi Arabia they announced on Monday. The group said they attacked two Saudi tankers with drones and missiles earlier on Thursday…