Fix clang-tidy warnings. (#4149)

* Upgrade gtest for clang-tidy.
* Use CMake to install GTest instead of mv.
* Don't enforce clang-tidy to return 0 due to errors in thrust.
* Add a small test for tidy itself.

* Reformat.

tests/cpp/data/test_data.cc

@@ -13,7 +13,7 @@ TEST(SparsePage, PushCSC) {
 
   offset = {0, 1, 4};
   for (size_t i = 0; i < offset.back(); ++i) {
-    data.push_back(Entry(i, 0.1f));
+    data.emplace_back(Entry(i, 0.1f));
   }
 
   SparsePage other;
@@ -52,4 +52,4 @@ TEST(SparsePage, PushCSC) {
     ASSERT_EQ(inst[i].index, indices_sol[i % 3]);
   }
 }
-}
+}  // namespace xgboost

tests/cpp/data/test_simple_dmatrix.cc

@@ -27,7 +27,7 @@ TEST(SimpleDMatrix, RowAccess) {
   xgboost::DMatrix * dmat = xgboost::DMatrix::Load(tmp_file, false, false);
 
   // Loop over the batches and count the records
-  long row_count = 0;
+  int64_t row_count = 0;
   for (auto &batch : dmat->GetRowBatches()) {
     row_count += batch.Size();
   }
@@ -54,7 +54,7 @@ TEST(SimpleDMatrix, ColAccessWithoutBatches) {
   ASSERT_TRUE(dmat->SingleColBlock());
 
   // Loop over the batches and assert the data is as expected
-  long num_col_batch = 0;
+  int64_t num_col_batch = 0;
   for (const auto &batch : dmat->GetSortedColumnBatches()) {
     num_col_batch += 1;
     EXPECT_EQ(batch.Size(), dmat->Info().num_col_)

tests/cpp/data/test_sparse_page_dmatrix.cc

@@ -1,6 +1,8 @@
 // Copyright by Contributors
 #include <xgboost/data.h>
 #include <dmlc/filesystem.h>
+#include <cinttypes>
+
 #include "../../../src/data/sparse_page_dmatrix.h"
 
 #include "../helpers.h"
@@ -33,7 +35,7 @@ TEST(SparsePageDMatrix, RowAccess) {
   EXPECT_TRUE(FileExists(tmp_file + ".cache.row.page"));
 
   // Loop over the batches and count the records
-  long row_count = 0;
+  int64_t row_count = 0;
   for (auto &batch : dmat->GetRowBatches()) {
     row_count += batch.Size();
   }