fix unit test

Author: 王俊博

flink-cdc-connect/flink-cdc-source-connectors/flink-connector-mysql-cdc/src/test/java/org/apache/flink/cdc/connectors/mysql/source/assigners/MySqlHybridSplitAssignerTest.java

@@ -119,6 +119,7 @@ public void testAssignMySqlBinlogSplitAfterAllSnapshotSplitsFinished() {
                         configuration, DEFAULT_PARALLELISM, checkpoint, enumeratorContext);
 
         // step 2. Get the MySqlBinlogSplit after all snapshot splits finished
+        assigner.open();
         Optional<MySqlSplit> binlogSplit = assigner.getNext();
         MySqlBinlogSplit mySqlBinlogSplit = binlogSplit.get().asBinlogSplit();