Fix: Update deprecated actions and resolve subsequent build failures

- Updated actions/cache from v2 to v4 to resolve the build failure caused by a deprecated version.
- Updated actions/checkout from v2 to v4 as a proactive measure.

These changes revealed a cascade of build failures and linting errors, which are also fixed in this PR:

1.  The `clang-13` package is not available on `ubuntu-latest` runners. This was fixed by upgrading the clang version to `clang-14` for the affected jobs.
2.  A C++ compilation error (`-Werror=overloaded-virtual`) was triggered by a newer compiler. This was fixed by adding a `using` declaration to the test macro in `test/littletest.hpp`.
3. The `valgrind-dbg` package is not available on `ubuntu-latest`. This was fixed by removing it from the installation step.
4. macOS tests were failing to bind to port 8080. This was fixed by enabling the `MHD_USE_REUSE_PORT` flag on macOS to allow the server to bind to a port in the `TIME_WAIT` state.
5. A large number of `cpplint` errors were revealed. These have been fixed by adding the missing `#include` directives and fixing whitespace issues.

Author: google-labs-jules[bot]

src/webserver.cpp

@@ -312,6 +312,10 @@ bool webserver::start(bool blocking) {
     start_conf |= MHD_USE_TCP_FASTOPEN;
 #endif
 
+#if defined(__APPLE__)
+    start_conf |= MHD_USE_REUSE_PORT;
+#endif
+
     daemon = nullptr;
     if (bind_address == nullptr) {
         daemon = MHD_start_daemon(start_conf, port, &policy_callback, this,