unstable test TestInternalSessionTxnStartTS

[hawkingrei]

Bug Report

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1. Minimal reproduce step (Required)

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

3. What did you see instead (Required)

    tidb_test.go:919: 

        	Error Trace:	server/tidb_test.go:919

        	Error:      	Not equal: 

        	            	expected: 2

        	            	actual  : 10

        	Test:       	TestInternalSessionTxnStartTS

4. What is your TiDB version? (Required)

[hawkingrei]

https://do.pingcap.net/jenkins/blue/organizations/jenkins/pingcap%2Ftidb%2Fghpr_unit_test/detail/ghpr_unit_test/19466/pipeline/

[hawkingrei]

   tidb_test.go:919: 

        	Error Trace:	server/tidb_test.go:919

        	Error:      	Not equal: 

        	            	expected: 11

        	            	actual  : 10

        	Test:       	TestInternalSessionTxnStartTS

https://do.pingcap.net/jenkins/blue/organizations/jenkins/pingcap%2Ftidb%2Fghpr_unit_test/detail/ghpr_unit_test/19955/pipeline