PARIS—The trial of 20 men accused of involvement in a jihadist rampage across Paris in 2015 began under high security in the French capital on Wednesday, with the man widely believed to be the only surviving attacker appearing in a black mask. Some 130 people were killed and hundreds wounded when gunmen with suicide vests attacked six bars and restaurants, the Bataclan concert hall, and a sports stadium on Nov. 13, 2015, leaving deep scars on the nation’s psyche. Police mounted tight security around the Palais de Justice courthouse in central Paris, and survivors and relatives of the victims said they were impatient to hear testimony that might help them better understand what happened and why it did so. The accused will appear behind a reinforced glass partition in a purpose-built courtroom. Among them was French-Moroccan Salah Abdeslam, 31, who arrived in the courtroom dressed all in black, including with …