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_iostream.cpp

@@ -34,7 +34,7 @@ void noisy_funct_dual(std::string msg, std::string emsg) {
 // simply repeatedly write to std::cerr until stopped
 // redirect is called at some point to test the safety of scoped_estream_redirect
 struct TestThread {
-    TestThread() : t_{nullptr}, stop_{false} {
+    TestThread() : stop_{false} {
         auto thread_f = [this] {
             while (!stop_) {
                 std::cout << "x" << std::flush;
@@ -49,7 +49,7 @@ struct TestThread {
 
     void stop() { stop_ = true; }
 
-    void join() {
+    void join() const {
         py::gil_scoped_release gil_lock;
         t_->join();
     }
@@ -59,7 +59,7 @@ struct TestThread {
         std::this_thread::sleep_for(std::chrono::milliseconds(50));
     }
 
-    std::thread * t_;
+    std::thread *t_{nullptr};
     std::atomic<bool> stop_;
 };