unstable test

[zhouqiang-cl]

Bug Report

https://ci.pingcap.net/blue/organizations/jenkins/tidb_ghpr_check_2/detail/tidb_ghpr_check_2/3743/pipeline/

[2021-05-12T10:09:55.888Z] FAIL: extract_start_ts_test.go:64: extractStartTsSuite.TestExtractStartTs
[2021-05-12T10:09:55.888Z] 
[2021-05-12T10:09:55.888Z] extract_start_ts_test.go:102:
[2021-05-12T10:09:55.889Z]     c.Assert(result, Equals, expected)
[2021-05-12T10:09:55.889Z] ... obtained uint64 = 0x5e5803123ac0000
[2021-05-12T10:09:55.889Z] ... expected uint64 = 0x5e5803123a80000
[2021-05-12T10:09:55.889Z] 
[2021-05-12T10:09:55.889Z] PASS: region_request_test.go:300: testRegionRequestToSingleStoreSuite.TestNoReloadRegionWhenCtxCanceled	0.000s
[2021-05-12T10:09:55.889Z] [2021/05/12 18:09:48.431 +08:00] [WARN] [region_request.go:720] ["tikv reports `MaxTimestampNotSynced`"] [ctx="region ID: 3, meta: id:3 peers:<id:2 store_id:1 > , peer: id:2 store_id:1 , addr: store1, idx: 0, reqStoreType: TiKvOnly, runStoreType: tikv"]
[2021-05-12T10:09:55.889Z] [2021/05/12 18:09:48.435 +08:00] [WARN] [region_request.go:720] ["tikv reports `MaxTimestampNotSynced`"] [ctx="region ID: 3, meta: id:3 peers:<id:2 store_id:1 > , peer: id:2 store_id:1 , addr: store1, idx: 0, reqStoreType: TiKvOnly, runStoreType: tikv"]
[2021-05-12T10:09:55.889Z] PASS: region_cache_test.go:197: testRegionCacheSuite.TestDropStoreRetry	0.010s

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run CI

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4. What is your TiDB version? (Required)

in #24408

[ti-srebot]

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