Use UBJSON for serializing splits for vertical data split. (#10059)

tests/cpp/tree/test_quantile_hist.cc

@@ -1,5 +1,5 @@
 /**
- * Copyright 2018-2023 by XGBoost Contributors
+ * Copyright 2018-2024, XGBoost Contributors
  */
 #include <gtest/gtest.h>
 #include <xgboost/host_device_vector.h>
@@ -18,7 +18,6 @@
 #include "xgboost/data.h"
 
 namespace xgboost::tree {
-
 namespace {
 template <typename ExpandEntry>
 void TestPartitioner(bst_target_t n_targets) {
@@ -253,5 +252,5 @@ void TestColumnSplit(bst_target_t n_targets) {
 
 TEST(QuantileHist, ColumnSplit) { TestColumnSplit(1); }
 
-TEST(QuantileHist, DISABLED_ColumnSplitMultiTarget) { TestColumnSplit(3); }
+TEST(QuantileHist, ColumnSplitMultiTarget) { TestColumnSplit(3); }
 }  // namespace xgboost::tree