test/e2e: fix WaitLeader backport

Fixes the incorrect backport of WaitLeader, as spawnJsonCmd receives an
expected output string as an argument, which wasn't provided in the
first backport implementation.

Original backport of commit 371179e from
PR etcd-io#17398.

Signed-off-by: Ivan Valdes <ivan@vald.es>

tests/e2e/etcdctl.go

@@ -146,7 +146,7 @@ func (ctl *Etcdctl) Status() ([]*clientv3.StatusResponse, error) {
 		Endpoint string
 		Status   *clientv3.StatusResponse
 	}
-	err := ctl.spawnJsonCmd(&epStatus, "endpoint", "status")
+	err := ctl.spawnJsonCmd(&epStatus, "", "endpoint", "status")
 	if err != nil {
 		return nil, err
 	}