Fixed errcheck issues in server/channels/app/export_test.go (#29004)

Co-authored-by: Ben Schumacher <ben.schumacher@mattermost.com>

server/.golangci.yml

@@ -84,7 +84,6 @@ issues:
         channels/app/brand.go|\
         channels/app/config_test.go|\
         channels/app/export.go|\
-        channels/app/export_test.go|\
         channels/app/file.go|\
         channels/app/file_bench_test.go|\
         channels/app/file_test.go|\