Fix some unstable complex delete test (milvus-io#27497)

Signed-off-by: aoiasd <zhicheng.yue@zilliz.com>

Author: aoiasd

tests/python_client/testcases/test_delete.py

@@ -2182,6 +2182,7 @@ def test_delete_string_expressions_normal(self, expression, enable_dynamic_field
             data = cf.gen_default_dataframe_data(nb)
             data[ct.default_json_field_name] = [{"string": str(i)} for i in range(nb)]
         collection_w.insert(data)
+        collection_w.flush()
         collection_w.load()
 
         # calculate the result