More tests for column split and vertical federated learning (#8985)

Added some more tests for the learner and fit_stump, for both column-wise distributed learning and vertical federated learning.

Also moved the `IsRowSplit` and `IsColumnSplit` methods from the `DMatrix` to the `MetaInfo` since in some places we only have access to the `MetaInfo`. Added a new convenience method `IsVerticalFederatedLearning`.

Some refactoring of the testing fixtures.

tests/cpp/helpers.h

@@ -101,9 +101,8 @@ double GetMultiMetricEval(xgboost::Metric* metric,
                           std::vector<xgboost::bst_uint> groups = {});
 
 namespace xgboost {
-bool IsNear(std::vector<xgboost::bst_float>::const_iterator _beg1,
-            std::vector<xgboost::bst_float>::const_iterator _end1,
-            std::vector<xgboost::bst_float>::const_iterator _beg2);
+
+float GetBaseScore(Json const &config);
 
 /*!
  * \brief Linear congruential generator.