Fix prediction heuristic (#5955)

* Relax check for prediction.
* Relax test in spark test.
* Add tests in C++.

tests/cpp/predictor/test_gpu_predictor.cu

@@ -160,5 +160,8 @@ TEST(GPUPredictor, MGPU_InplacePredict) {  // NOLINT
                dmlc::Error);
 }
 
+TEST(GpuPredictor, LesserFeatures) {
+  TestPredictionWithLesserFeatures("gpu_predictor");
+}
 }  // namespace predictor
 }  // namespace xgboost