Running Tests Creates ModuleNotFoundError for Demos in DataManager

[mdr223]

After running tests with pytest -v tests/pytest, I've noticed that if I then run a demo script I'll see an error like:

(venv) matthewrusso@Matthews-MBP-2 palimpzest % python demos/imageDemo.py 
Traceback (most recent call last):
  File "/Users/matthewrusso/tmp/palimpzest/demos/imageDemo.py", line 36, in <module>
    if datasetid not in pz.DataDirectory().listRegisteredDatasets():
                        ^^^^^^^^^^^^^^^^^^
  File "/Users/matthewrusso/tmp/palimpzest/src/palimpzest/datamanager/datamanager.py", line 27, in __call__
    instance = super(DataDirectorySingletonMeta, cls).__call__(
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/matthewrusso/tmp/palimpzest/src/palimpzest/datamanager/datamanager.py", line 93, in __init__
    self._registry = pickle.load(f)
                     ^^^^^^^^^^^^^^
ModuleNotFoundError: No module named 'fixtures'

Making a formal issue as a reminder to resolve this in the near-term.