Wait for async mutation in test

tests/integration/test_version_update_after_mutation/test.py

 import pytest
-import time
 
 from helpers.cluster import ClickHouseCluster
 from helpers.test_tools import assert_eq_with_retry
@@ -71,4 +70,7 @@ def test_upgrade_while_mutation(start_cluster):
 
     node3.restart_with_latest_version()
 
+    # will delete nothing, but previous async mutation will finish with this query
+    node3.query("ALTER TABLE mt1 DELETE WHERE id > 100000", settings={"mutations_sync": "2"})
+
     assert_eq_with_retry(node3, "SELECT COUNT() from mt1", "50000\n")