Improve test of csvstack --filenames.
Improve test of csvstack --filenames.

wjr1985/csvkit,cypreess/csvkit,themiurgo/csvkit,doganmeh/csvkit,matterker/csvkit,kyeoh/csvkit,Tabea-K/csvkit,arowla/csvkit,aequitas/csvkit,nriyer/csvkit,tlevine/csvkit,haginara/csvkit,jpalvarezf/csvkit,Jobava/csvkit,KarrieK/csvkit,barentsen/csvkit,onyxfish/csvkit,dannguyen/csvkit,elcritch/csvkit,snuggles08/csvkit,reubano/csvkit,wireservice/csvkit,metasoarous/csvkit,gepuro/csvkit,archaeogeek/csvkit,moradology/csvkit,bradparks/csvkit__query_join_filter_CSV_cli,unpingco/csvkit,bmispelon/csvkit