ddl: re-add drop column test.
ddl: re-add drop column test.

liyu4/tidb,kyle-wang/tidb,coocood/tidb,astaxie/tidb,c4pt0r/tidb,wusphinx/tidb,kyle-wang/tidb,zs634134578/tidb,andelf/tidb,Cofyc/tidb,pingcap/tidb,Nify/tidb,CodeRushing/tidb,ngaut/tidb,zs634134578/tidb,ngaut/tidb,c4pt0r/tidb,pingcap/tidb,spongedu/tidb,cosmtrek/tidb,ming-relax/tidb,ueizhou/tidb,wusphinx/tidb,XuHuaiyu/tidb,CodeRushing/tidb,yuanwhy/tidb,anywhy/tidb,simon-xia/tidb,pingcap/tidb,pingcap/tidb,8cbx/tidb,JinheLin/tidb,tiancaiamao/tidb,kyle-wang/tidb,XuHuaiyu/tidb,Nify/tidb,kyle-wang/tidb,silentred/tidb,tiancaiamao/tidb,qiuyesuifeng/tidb,astaxie/tidb,myrice/tidb,c4pt0r/tidb,pingcap/tidb,coocood/tidb,simon-xia/tidb,anywhy/tidb,zimulala/tidb,liyu4/tidb,c4pt0r/tidb,silenceper/tidb,ngaut/tidb,qhsong/tidb,mccxj/tidb,mccxj/tidb,ngaut/tidb,EvilMcJerkface/tidb,louishust/tidb,ngaut/tidb,yuanwhy/tidb,gaocegege/tidb,EvilMcJerkface/tidb,spongedu/tidb,zimulala/tidb,Cofyc/tidb,yangwenmai/tidb,silentred/tidb,kyle-wang/tidb,ming-relax/tidb,gaocegege/tidb,kyle-wang/tidb,JinheLin/tidb,cosmtrek/tidb,c4pt0r/tidb,8cbx/tidb,silenceper/tidb,louishust/tidb,qiuyesuifeng/tidb,c4pt0r/tidb,andelf/tidb,yangwenmai/tidb,qhsong/tidb,ueizhou/tidb,ngaut/tidb,pingcap/tidb,myrice/tidb