ddl: stable placement cache test (#20770)
ddl: stable placement cache test (#20770)

Signed-off-by: xhe <3cc054ec827480b50ed66e70bec75f34d9e09619@gmail.com>
ngaut/tidb,pingcap/tidb,c4pt0r/tidb,pingcap/tidb,c4pt0r/tidb,kyle-wang/tidb,ngaut/tidb,kyle-wang/tidb,pingcap/tidb,c4pt0r/tidb,coocood/tidb,ngaut/tidb,ngaut/tidb,coocood/tidb,c4pt0r/tidb,c4pt0r/tidb,kyle-wang/tidb,XuHuaiyu/tidb,kyle-wang/tidb,pingcap/tidb,pingcap/tidb,ngaut/tidb,c4pt0r/tidb,kyle-wang/tidb,EvilMcJerkface/tidb,pingcap/tidb,EvilMcJerkface/tidb,ngaut/tidb,kyle-wang/tidb,XuHuaiyu/tidb