message: [gha][lbt] append a happy message when LBT passes
  If LBT passes, add a line that says:
  tada: Land-blocking test passed! :ok_hand:
  Closes:
diff: @@ -296,6 +296,9 @@ jobs:
  }
  // Post test result on original pull request
  try {
  + if (!should_fail) {
  + body += "\n :tada: Land-blocking cluster test passed! :ok_hand:"
  + }
  await github.issues.createComment(
  {
  issue_number: pr_num,
  