Fix errcheck issues in server/channels/app/plugin_deadlock_test.go (#29113)

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

server/.golangci.yml

@@ -133,7 +133,6 @@ issues:
         channels/app/plugin_api_tests/manual.test_http_hijack_plugin/main.go|\
         channels/app/plugin_api_tests/manual.test_serve_metrics_plugin/main.go|\
         channels/app/plugin_api_tests/test_update_user_auth_plugin/main.go|\
-        channels/app/plugin_deadlock_test.go|\
         channels/app/plugin_health_check_test.go|\
         channels/app/plugin_install.go|\
         channels/app/plugin_requests.go|\