refactor tests to get rid of duplication (#4358)

* refactor tests to get rid of duplication

* address review comments

tests/cpp/helpers.cc

@@ -5,6 +5,7 @@
 #include "xgboost/c_api.h"
 #include <random>
 #include <cinttypes>
+#include <dmlc/filesystem.h>
 
 bool FileExists(const std::string& filename) {
   struct stat st;
@@ -142,4 +143,38 @@ std::shared_ptr<xgboost::DMatrix>* CreateDMatrix(int rows, int columns,
   return static_cast<std::shared_ptr<xgboost::DMatrix> *>(handle);
 }
 
+std::unique_ptr<DMatrix> CreateSparsePageDMatrix() {
+  // Create sufficiently large data to make two row pages
+  dmlc::TemporaryDirectory tempdir;
+  const std::string tmp_file = tempdir.path + "/big.libsvm";
+  CreateBigTestData(tmp_file, 12);
+  std::unique_ptr<DMatrix> dmat = std::unique_ptr<DMatrix>(DMatrix::Load(
+      tmp_file + "#" + tmp_file + ".cache", true, false, "auto", 64UL));
+  EXPECT_TRUE(FileExists(tmp_file + ".cache.row.page"));
+
+  // Loop over the batches and count the records
+  int64_t batch_count = 0;
+  int64_t row_count = 0;
+  for (const auto &batch : dmat->GetRowBatches()) {
+    batch_count++;
+    row_count += batch.Size();
+  }
+  EXPECT_EQ(batch_count, 2);
+  EXPECT_EQ(row_count, dmat->Info().num_row_);
+
+  return dmat;
+}
+
+gbm::GBTreeModel CreateTestModel() {
+  std::vector<std::unique_ptr<RegTree>> trees;
+  trees.push_back(std::unique_ptr<RegTree>(new RegTree));
+  (*trees.back())[0].SetLeaf(1.5f);
+  (*trees.back()).Stat(0).sum_hess = 1.0f;
+  gbm::GBTreeModel model(0.5);
+  model.CommitModel(std::move(trees), 0);
+  model.param.num_output_group = 1;
+  model.base_margin = 0;
+  return model;
+}
+
 }  // namespace xgboost