[MM-61465] Fix errcheck issues in server/channels/app/post_metadata_test.go (#29206)

server/.golangci.yml

@@ -135,7 +135,6 @@ issues:
         channels/app/post.go|\
         channels/app/post_helpers_test.go|\
         channels/app/post_metadata.go|\
-        channels/app/post_metadata_test.go|\
         channels/app/post_persistent_notification_test.go|\
         channels/app/post_test.go|\
         channels/app/product_notices_test.go|\