expression/agg: add unit-test. (#6074)
expression/agg: add unit-test. (#6074)


kyle-wang/tidb,coocood/tidb,kyle-wang/tidb,ngaut/tidb,c4pt0r/tidb,coocood/tidb,XuHuaiyu/tidb,Nify/tidb,pingcap/tidb,Cofyc/tidb,pingcap/tidb,pingcap/tidb,tiancaiamao/tidb,kyle-wang/tidb,ngaut/tidb,c4pt0r/tidb,c4pt0r/tidb,kyle-wang/tidb,c4pt0r/tidb,mccxj/tidb,qiuyesuifeng/tidb,ngaut/tidb,Nify/tidb,cosmtrek/tidb,pingcap/tidb,wusphinx/tidb,tiancaiamao/tidb,cosmtrek/tidb,ngaut/tidb,c4pt0r/tidb,pingcap/tidb,ming-relax/tidb,ming-relax/tidb,mccxj/tidb,qiuyesuifeng/tidb,pingcap/tidb,kyle-wang/tidb,wusphinx/tidb,CodeRushing/tidb,EvilMcJerkface/tidb,c4pt0r/tidb,EvilMcJerkface/tidb,kyle-wang/tidb,ngaut/tidb,ngaut/tidb,Cofyc/tidb,CodeRushing/tidb,XuHuaiyu/tidb