More support for column split in cpu predictor (#9244)

- Added column split support to `PredictInstance` and `PredictLeaf`.
- Refactoring of tests.

include/xgboost/predictor.h

@@ -134,16 +134,18 @@ class Predictor {
    * usually more efficient than online prediction This function is NOT
    * threadsafe, make sure you only call from one thread.
    *
-   * \param           inst        The instance to predict.
-   * \param [in,out]  out_preds   The output preds.
-   * \param           model       The model to predict from
-   * \param           tree_end    (Optional) The tree end index.
+   * \param           inst            The instance to predict.
+   * \param [in,out]  out_preds       The output preds.
+   * \param           model           The model to predict from
+   * \param           tree_end        (Optional) The tree end index.
+   * \param           is_column_split (Optional) If the data is split column-wise.
    */
 
   virtual void PredictInstance(const SparsePage::Inst& inst,
                                std::vector<bst_float>* out_preds,
                                const gbm::GBTreeModel& model,
-                               unsigned tree_end = 0) const = 0;
+                               unsigned tree_end = 0,
+                               bool is_column_split = false) const = 0;
 
   /**
    * \brief predict the leaf index of each tree, the output will be nsample *