[CI] Add noLD R test (#6382)

* [CI] Add noLD test

* Make noLD test only trigger with a PR comment

* [CI] Don't install stringi

* Add the Titanic example as a unit test

* Document trigger

* add to index

* Clarify that it needs to be a review comment

R-package/tests/testthat/test_callbacks.R

@@ -2,6 +2,7 @@
 
 require(xgboost)
 require(data.table)
+require(titanic)
 
 context("callbacks")
 
@@ -254,6 +255,26 @@ test_that("early stopping using a specific metric works", {
   expect_equal(logloss_log, logloss_pred, tolerance = 1e-5)
 })
 
+test_that("early stopping works with titanic", {
+  # This test was inspired by https://github.com/dmlc/xgboost/issues/5935
+  # It catches possible issues on noLD R
+  titanic <- titanic::titanic_train
+  titanic$Pclass <-  as.factor(titanic$Pclass)
+  dtx <- model.matrix(~ 0 + ., data = titanic[, c("Pclass", "Sex")])
+  dty <- titanic$Survived
+
+  xgboost::xgboost(
+    data = dtx,
+    label = dty,
+    objective = "binary:logistic",
+    eval_metric = "auc",
+    nrounds = 100,
+    early_stopping_rounds = 3
+  )
+
+  expect_true(TRUE)  # should not crash
+})
+
 test_that("early stopping xgb.cv works", {
   set.seed(11)
   expect_output(