refactor tests to get rid of duplication (#4358)

* refactor tests to get rid of duplication

* address review comments

tests/cpp/helpers.h

@@ -17,6 +17,7 @@
 #include <xgboost/base.h>
 #include <xgboost/objective.h>
 #include <xgboost/metric.h>
+#include <xgboost/predictor.h>
 
 #if defined(__CUDACC__)
 #define DeclareUnifiedTest(name) GPU ## name
@@ -153,5 +154,9 @@ class SimpleRealUniformDistribution {
 std::shared_ptr<xgboost::DMatrix> *CreateDMatrix(int rows, int columns,
                                                  float sparsity, int seed = 0);
 
+std::unique_ptr<DMatrix> CreateSparsePageDMatrix();
+
+gbm::GBTreeModel CreateTestModel();
+
 }  // namespace xgboost
 #endif