distsql: improve test coverage for "request_builder_test.go" (#5908)
distsql: improve test coverage for "request_builder_test.go" (#5908)


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