br: fix race in test `Test(Split|Restore)Failed` (#33313)
br: fix race in test `Test(Split|Restore)Failed` (#33313)

close pingcap/tidb#33310
c4pt0r/tidb,ngaut/tidb,c4pt0r/tidb,c4pt0r/tidb,pingcap/tidb,ngaut/tidb,c4pt0r/tidb,c4pt0r/tidb,ngaut/tidb,kyle-wang/tidb,ngaut/tidb,kyle-wang/tidb,kyle-wang/tidb,ngaut/tidb,c4pt0r/tidb,pingcap/tidb,pingcap/tidb,kyle-wang/tidb,pingcap/tidb,ngaut/tidb,pingcap/tidb,pingcap/tidb,kyle-wang/tidb,kyle-wang/tidb