The legions of protesters demonstrating at a leaders summit here have unfurled a sea of standard-issue signs, ranging from “No to Imperialism” to “Yankees Go Home.” But a newer rallying cry also appeared on a smattering of homemade posters. “Mohammed bin Salman, Assassin!” For Saudi Arabian Crown Prince Mohammed bin Salman, the Group of 20 leaders summit in Buenos Aires has amounted to a key test: his first appearance at a major international event since the killing of dissident journalist Jamal Khashoggi, a Saudi who was a frequent critic of the kingdom’s de facto leader.