Use `UpdateAllowUnknown' for non-model related parameter. (#4961)

* Use `UpdateAllowUnknown' for non-model related parameter.

Model parameter can not pack an additional boolean value due to binary IO
format.  This commit deals only with non-model related parameter configuration.

* Add tidy command line arg for use-dmlc-gtest.

tests/cpp/tree/test_prune.cc

@@ -33,7 +33,7 @@ TEST(Updater, Prune) {
 
   // prepare tree
   RegTree tree = RegTree();
-  tree.param.InitAllowUnknown(cfg);
+  tree.param.UpdateAllowUnknown(cfg);
   std::vector<RegTree*> trees {&tree};
   // prepare pruner
   std::unique_ptr<TreeUpdater> pruner(TreeUpdater::Create("prune", &lparam));

tests/cpp/tree/test_quantile_hist.cc

@@ -262,7 +262,7 @@ class QuantileHistMock : public QuantileHistMaker {
     gmat.Init((*dmat_).get(), kMaxBins);
 
     RegTree tree = RegTree();
-    tree.param.InitAllowUnknown(cfg_);
+    tree.param.UpdateAllowUnknown(cfg_);
 
     std::vector<GradientPair> gpair =
         { {0.23f, 0.24f}, {0.23f, 0.24f}, {0.23f, 0.24f}, {0.23f, 0.24f},
@@ -273,7 +273,7 @@ class QuantileHistMock : public QuantileHistMaker {
 
   void TestBuildHist() {
     RegTree tree = RegTree();
-    tree.param.InitAllowUnknown(cfg_);
+    tree.param.UpdateAllowUnknown(cfg_);
 
     size_t constexpr kMaxBins = 4;
     common::GHistIndexMatrix gmat;
@@ -284,7 +284,7 @@ class QuantileHistMock : public QuantileHistMaker {
 
   void TestEvaluateSplit() {
     RegTree tree = RegTree();
-    tree.param.InitAllowUnknown(cfg_);
+    tree.param.UpdateAllowUnknown(cfg_);
 
     builder_->TestEvaluateSplit(gmatb_, tree);
   }

tests/cpp/tree/test_refresh.cc

@@ -28,7 +28,7 @@ TEST(Updater, Refresh) {
 
   RegTree tree = RegTree();
   auto lparam = CreateEmptyGenericParam(GPUIDX);
-  tree.param.InitAllowUnknown(cfg);
+  tree.param.UpdateAllowUnknown(cfg);
   std::vector<RegTree*> trees {&tree};
   std::unique_ptr<TreeUpdater> refresher(TreeUpdater::Create("refresh", &lparam));