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/common/test_span.cc

@@ -172,7 +172,7 @@ struct BaseClass {
   virtual void operator()() {}
 };
 struct DerivedClass : public BaseClass {
-  virtual void operator()() {}
+  void operator()() override {}
 };
 
 TEST(Span, FromOther) {