UT unstable test case TestInvalidReadCacheTable

[tiancaiamao]

Bug Report

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1. Minimal reproduce step (Required)

Recently reproduce many time in CI

https://ci.pingcap.net/blue/organizations/jenkins/tidb_ghpr_unit_test/detail/tidb_ghpr_unit_test/38976/pipeline/

[2022-03-04T03:58:22.683Z] ----------------------------------------------------------------------
[2022-03-04T03:58:22.683Z] FAIL: executor_test.go:9077: testStaleTxnSuite.TestInvalidReadCacheTable
[2022-03-04T03:58:22.683Z] 
[2022-03-04T03:58:22.683Z] executor_test.go:9158:
[2022-03-04T03:58:22.683Z]     tk.MustQuery("select * from cache_tmp5 as of timestamp(@a) where id=1;").Check(testkit.Rows("1"))
[2022-03-04T03:58:22.683Z] /home/jenkins/agent/workspace/tidb_ghpr_unit_test/go/src/github.com/pingcap/tidb/util/testkit/testkit.go:67:
[2022-03-04T03:58:22.683Z]     res.c.Assert(resBuff.String(), check.Equals, needBuff.String(), res.comment)
[2022-03-04T03:58:22.684Z] ... obtained string = ""
[2022-03-04T03:58:22.684Z] ... expected string = "[1]\n"
[2022-03-04T03:58:22.684Z] ... sql:select * from cache_tmp5 as of timestamp(@a) where id=1;, args:[]

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

CI success

3. What did you see instead (Required)

Failed

4. What is your TiDB version? (Required)

master 74d74b0