flaky test in the flashback_test

[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)

panic: region 40 is in flashback progress, FlashbackStartTS is 449658828228919302



goroutine 175816 [running]:

github.com/pingcap/tidb/pkg/infoschema.(*infoschemaV2).SchemaTables(0xc018854c00, {{0xc01fb590fc?, 0xc007625680?}, {0xc01fb5910c?, 0xc021880180?}})

	pkg/infoschema/infoschema_v2.go:600 +0x512

github.com/pingcap/tidb/pkg/ddl.(*ddl).refreshTiFlashTicker(0xc0207c50e0, {0xa070590, 0xc007625680}, 0xc021880180)

	pkg/ddl/ddl_tiflash_api.go:455 +0x623

github.com/pingcap/tidb/pkg/ddl.(*ddl).PollTiFlashRoutine(0xc0207c50e0)

	pkg/ddl/ddl_tiflash_api.go:588 +0x9c5

github.com/pingcap/tidb/pkg/util.(*WaitGroupWrapper).Run.func1()

	pkg/util/wait_group_wrapper.go:157 +0x9e

created by github.com/pingcap/tidb/pkg/util.(*WaitGroupWrapper).Run in goroutine 175139

	pkg/util/wait_group_wrapper.go:155 +0xf1

4. What is your TiDB version? (Required)

[Defined2014]

Caused by #52896, PTAL @tiancaiamao

[tiancaiamao]

Only affect master branch