Look into hanging unit tests

[wyld-sw]

The following command during build checks does not seem to exit:

cd tests && python3 -m unittest -b

Some results:

/home/runner/work/fuzzball/fuzzball/tests/test_util.py:334: YAMLLoadWarning: calling yaml.load() without Loader=... is deprecated, as the default Loader is unsafe. Please read https://msg.pyyaml.org/load for full details.
  lst = yaml.load(fh)
E
Stderr:
Loop <_UnixSelectorEventLoop running=False closed=True debug=False> that handles pid 2494 is closed
E
Stderr:
Loop <_UnixSelectorEventLoop running=False closed=True debug=False> that handles pid 2521 is closed
E
...