[test] Record and assert thread failures in the overload-reuse test

[aaronj0]

As seen on the CI on mac platforms:

[93](https://github.com/compiler-research/cppjit/actions/runs/34221110946/job/102044181415#step:8:702)
    =============================== warnings summary ===============================
    test/test_concurrent.py::TestCONCURRENT::test06_overload_reuse_in_threads
      /Library/Frameworks/Python.framework/Versions/3.12/lib/python3.12/site-packages/_pytest/threadexception.py:58: PytestUnhandledThreadExceptionWarning: Exception in thread Thread-104 (test)
      
      Traceback (most recent call last):
        File "/Library/Frameworks/Python.framework/Versions/3.12/lib/python3.12/threading.py", line 1075, in _bootstrap_inner
          self.run()
        File "/Library/Frameworks/Python.framework/Versions/3.12/lib/python3.12/threading.py", line 1012, in run
          self._target(*self._args, **self._kwargs)
        File "/Users/runner/work/cppjit/cppjit/test/test_concurrent.py", line 257, in test
          simulation.set_something(o, ".")
      TypeError: void CPPOverloadReuse::Simulation1::set_something(std::map<std::string, std::string>, std::string) =>
          TypeError: could not convert argument 1: [InstanceConverter]
      
      Enable tracemalloc to get traceback where the object was allocated.
      See https://docs.pytest.org/en/stable/how-to/capture-warnings.html#resource-warnings for more info.
        warnings.warn(pytest.PytestUnhandledThreadExceptionWarning(msg))

the threads here failed but the test never checked if they raised an exception. This should expose the real failures