`certs_dir()` creates `self._cert_dir` as a `TemporaryDirectory`, but `cleanup()` never cleans it up. This leaks temp directories over multiple runs. Extend `cleanup()` to also cleanup `self._cert_dir` (and optionally reset cached traces if they hold resources).

[marten-seemann]

certs_dir() creates self._cert_dir as a TemporaryDirectory, but cleanup() never cleans it up. This leaks temp directories over multiple runs. Extend cleanup() to also cleanup self._cert_dir (and optionally reset cached traces if they hold resources).

            self._server_download_dir = None
        if hasattr(self, "_cert_dir") and self._cert_dir:
            self._cert_dir.cleanup()
            self._cert_dir = None

Originally posted by @Copilot in #464 (comment)