flaky test TestStaleReadProcessorWithExecutePreparedStmt

[hawkingrei]

Bug Report

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1. Minimal reproduce step (Required)

    processor_test.go:287: 
        	Error Trace:	/home/jenkins/.tidb/tmp/63a9840cd0739f2c243bb46478607469/sandbox/linux-sandbox/6624/execroot/__main__/bazel-out/k8-fastbuild/bin/sessiontxn/staleread/staleread_test_/staleread_test.runfiles/__main__/sessiontxn/staleread/processor_test.go:287
        	Error:      	Not equal: 
        	            	expected: 0
        	            	actual  : 15
        	Test:       	TestStaleReadProcessorWithExecutePreparedStmt

https://do.pingcap.net/jenkins/blue/rest/organizations/jenkins/pipelines/pingcap/pipelines/tidb/pipelines/ghpr_unit_test/runs/10027/nodes/49/steps/54/log/?start=0

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

3. What did you see instead (Required)

4. What is your TiDB version? (Required)