Tracebacks in test output

When running the test suite under `unittest`, there are tracebacks printed to the console. We should discover why those tracebacks are being printed, and either suppress the output (if it's harmless), or fix it (if not).

Steps to reproduce:

1. Create and activate a new Python 3.11 venv with `python -m venv --clear ~/.venvs/scimath && source ~/.venvs/scimath/bin/activate` (with the venv location of your choice, of course)
2. Install `scimath` into the venv with `python -m pip install -e .`
3. Run the test suite with `python -m unittest`

For me, this gives the following output:
```
(scimath) mdickinson@mirzakhani scimath % python -m unittest
1d block_average_above (sec): 8.917192462831736e-05
.scipy interp1d (sec): 0.013110005063936114
.1d interp (sec): 0.00018173002172261477
.fast interpolate (sec): 0.012866706005297601
..........................................................................................................................................................................................................................................................................................................................................................................................Unrecognized unit system m
Traceback (most recent call last):
File "/Users/mdickinson/Enthought/ETS/scimath/scimath/units/unit_manager.py", line 207, in get_unit_system
raise Exception
Exception
Unrecognized unit system m
Traceback (most recent call last):
File "/Users/mdickinson/Enthought/ETS/scimath/scimath/units/unit_manager.py", line 207, in get_unit_system
raise Exception
Exception
.....................
----------------------------------------------------------------------
Ran 402 tests in 0.337s

OK
```