Add fixture to close automatically Matplotlib figures in tests

[jhlegarreta]

What would you like to see added in this software?

Matplotlib figures in tests are most commonly not closed. If an excessive amount of figures is opened and not serialized, unless explicitly closed Matplotlib will raise a warning.

PR #167 proposed to explicitly close figures adding an extra plt.close("all") statement at the end of the relevant tests.

It was suggested that a fixture would be a more effective way to achieve this goal.

Do you have any interest in helping implement the feature?

Yes

Additional information / screenshots

N/A.