test/e2e: fix WaitLeader backport

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

Original backport of commit 371179e from
PR etcd-io#14752.

Signed-off-by: Ivan Valdes <[email protected]>

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
 	}