Upgrade clang-tidy on CI. (#5469)

* Correct all clang-tidy errors.
* Upgrade clang-tidy to 10 on CI.

Co-authored-by: Hyunsu Cho <chohyu01@cs.washington.edu>

tests/cpp/common/test_json.cc

@@ -262,7 +262,7 @@ TEST(Json, Indexing) {
   Json j {Json::Load(&reader)};
   auto& value_1 = j["model_parameter"];
   auto& value = value_1["base_score"];
-  std::string result = Cast<JsonString>(&value.GetValue())->getString();
+  std::string result = Cast<JsonString>(&value.GetValue())->GetString();
 
   ASSERT_EQ(result, "0.5");
 }
@@ -406,7 +406,7 @@ TEST(Json, WrongCasts) {
   }
 }
 
-TEST(Json, Int_vs_Float) {
+TEST(Json, IntVSFloat) {
   // If integer is parsed as float, calling `get<Integer>()' will throw.
   {
     std::string str = R"json(