unstable test case TestTiDBLastTxnInfoCommitMode

[XuHuaiyu]

[2021-05-20T02:22:49.872Z] FAIL: executor_test.go:3033: testSerialSuite.TestTiDBLastTxnInfoCommitMode
[2021-05-20T02:22:49.872Z] 
[2021-05-20T02:22:49.872Z] [["async_commit" false false]]
[2021-05-20T02:22:49.872Z] [["async_commit" false false]]
[2021-05-20T02:22:49.872Z] executor_test.go:3079:
[2021-05-20T02:22:49.872Z]     c.Assert(rows[0][0], Equals, `"2pc"`)
[2021-05-20T02:22:49.872Z] ... obtained string = "\"async_commit\""
[2021-05-20T02:22:49.872Z] ... expected string = "\"2pc\""

https://ci.pingcap.net/blue/organizations/jenkins/tidb_ghpr_unit_test/detail/tidb_ghpr_unit_test/3315/pipeline/103

[ti-srebot]

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