Fix traceback construction in tests using threadpool

[fperrin]

Fix issue #127

In a thread, the pseudo-traceback looks like:

_________ test_threadpool __________
FAILURE: check 2 == 3: 1 + 1 is 2, not 3
../../../../usr/local/Cellar/[email protected]/3.11.3/Frameworks/Python.framework/Versions/3.11/lib/python3.11/threading.py:995 in _bootstrap() -> self._bootstrap_inner()
../../../../usr/local/Cellar/[email protected]/3.11.3/Frameworks/Python.framework/Versions/3.11/lib/python3.11/threading.py:1038 in _bootstrap_inner() -> self.run()
../../../../usr/local/Cellar/[email protected]/3.11.3/Frameworks/Python.framework/Versions/3.11/lib/python3.11/threading.py:975 in run() -> self._target(*self._args, **self._kwargs)
../../../../usr/local/Cellar/[email protected]/3.11.3/Frameworks/Python.framework/Versions/3.11/lib/python3.11/concurrent/futures/thread.py:83 in _worker() -> work_item.run()
../../../../usr/local/Cellar/[email protected]/3.11.3/Frameworks/Python.framework/Versions/3.11/lib/python3.11/concurrent/futures/thread.py:58 in run() -> result = self.fn(*self.args, **self.kwargs)
examples/test_example_fail_in_thread.py:9 in force_fail() -> do_work(comparison)
examples/test_example_fail_in_thread.py:12 in do_work() -> check.equal(1 + 1, comparison, f"1 + 1 is 2, not {comparison}")

[okken]

Nice solution to this issue.

[fperrin]

Thank you @okken !