Fix test harness timeout enforcement
Fixes #6881.
`timeout_decorator()` used a `ThreadPoolExecutor` context manager around blocking output waits. When `future.result(timeout=...)` timed out, leaving the context manager still waited for the worker thread to finish, so a blocked stdout read could keep the instrumented tests hanging past the configured timeout.
This change removes that executor wrapper for interactive Stockfish output waits. The harness now drains process output on a daemon reader thread, queues received lines, and applies the deadline directly while waiting for the next queued line. `TimeoutException` also initializes the base exception message so failures show useful text.
Validation:
- `python3 -m py_compile tests/testing.py tests/instrumented.py`
- local timeout smoke test: a 0.2s no-output wait raises in ~0.204s
- Stockfish smoke test: startup/`uciok` read succeeds, deliberate no-output wait raises in ~0.205s, engine exits 0
- `make -C src -j4 build`
- `../tests/signature.sh` -> `2814421`
closes https://github.com/official-stockfish/Stockfish/pull/6882
No functional change