fix(clang-tidy): clang-tidy readability and misc fixes, like adding const (#3052)

* Enable and apply clang-tidy readability and misc fixes.

* Revert deprecated tester

* add space to tests/test_constants_and_functions.cpp

tests/test_smart_ptr.cpp

@@ -240,7 +240,7 @@ struct ElementBase {
 
 struct ElementA : ElementBase {
     ElementA(int v) : v(v) { }
-    int value() { return v; }
+    int value() const { return v; }
     int v;
 };