session: fix unit test TestUnionScanForMemBufferReader (#21258)
session: fix unit test TestUnionScanForMemBufferReader (#21258)


ngaut/tidb,c4pt0r/tidb,kyle-wang/tidb,kyle-wang/tidb,ngaut/tidb,coocood/tidb,kyle-wang/tidb,pingcap/tidb,pingcap/tidb,coocood/tidb,kyle-wang/tidb,c4pt0r/tidb,c4pt0r/tidb,ngaut/tidb,pingcap/tidb,c4pt0r/tidb,pingcap/tidb,ngaut/tidb,kyle-wang/tidb,c4pt0r/tidb,pingcap/tidb,kyle-wang/tidb,c4pt0r/tidb,pingcap/tidb,ngaut/tidb,ngaut/tidb