[MM-61460] Fix errcheck issues in server/channels/app/plugin_signature_test.go (#29348)

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

server/.golangci.yml

@@ -107,7 +107,6 @@ issues:
         channels/app/platform/web_conn.go|\
         channels/app/platform/web_hub.go|\
         channels/app/platform/web_hub_test.go|\
-        channels/app/plugin_signature_test.go|\
         channels/app/plugin_test.go|\
         channels/app/post_helpers_test.go|\
         channels/app/post_test.go|\