flaky test TestSwitchTaskStep

[hawkingrei]

Bug Report

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2. What did you expect to see? (Required)

3. What did you see instead (Required)

     table_test.go:211: 
        	Error Trace:	pkg/disttask/framework/storage/table_test.go:211
        	Error:      	Received unexpected error:
        	            	[kv:1062]Duplicate entry '1-1-1' for key 'tidb_background_subtask.uk_task_key_step_ordinal'
        	Test:       	TestSwitchTaskStep 

https://tiprow.hawkingrei.com/view/gs/pingcapprow/pr-logs/pull/pingcap_tidb/49951/tiprow_fast_test/1742148040417873920#1:build-log.txt%3A2259

4. What is your TiDB version? (Required)