unstable TestAddIndexIngestMemoryUsage

[lance6716]

Bug Report

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1. Minimal reproduce step (Required)

https://github.com/pingcap/tidb/pull/45471

ci.log

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

3. What did you see instead (Required)

    integration_test.go:66: 
        	Error Trace:	tests/realtikvtest/addindextest/integration_test.go:66
        	Error:      	Not equal: 
        	            	expected: 0
        	            	actual  : 176
        	Test:       	TestAddIndexIngestMemoryUsage

And I found two create one backend for an DDL job while one close one backend for DDL job

4. What is your TiDB version? (Required)