Restore NotifyListen to avoid panic in newServer retry (#6200)

agent/consul/server_test.go

@@ -183,6 +183,10 @@ func newServer(c *Config) (*Server, error) {
 			oldNotify()
 		}
 	}
+	// Restore old notify to guard against re-closing `up` on a retry
+	defer func() {
+		c.NotifyListen = oldNotify
+	}()
 
 	// start server
 	w := c.LogOutput