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.

src/common/hist_util.h

@@ -38,6 +38,7 @@ struct HistCutMatrix {
   void Init(std::vector<WXQSketch>* sketchs, uint32_t max_num_bins);
 
   HistCutMatrix();
+  size_t NumBins() const { return row_ptr.back(); }
 
  protected:
   virtual size_t SearchGroupIndFromBaseRow(