unit tests not closing figures

[jasmainak]

We should close the matplotlib figures, otherwise warnings such as these start coming:

/home/runner/work/hnn-core/hnn-core/hnn_core/viz.py:251: RuntimeWarning: More than 20 figures have been opened. Figures created through the pyplot interface (`matplotlib.pyplot.figure`) are retained until explicitly closed and may consume too much memory. (To control this warning, see the rcParam `figure.max_open_warning`).

It has a performance hit as well