test_proc_reader_send_signit is unreliable on Ubuntu for Python 3.12 & 3.13

The [test_proc_reader_send_signit](https://github.com/python-cmd2/cmd2/blob/master/tests/test_utils.py#L289-L305) unit test in `tests/test_utils.py` has intermittent reliability issues on Ubuntu for Python 3.12 and 3.13.

We have seen intermittent failures with this test on Ubuntu when using Python 3.12 or 3.13.  We have not seen failures for other OSes or versions of Python.