Add tests to improve code coverage of viz module 🧪

[sduquemesa]

This issue has been tagged for a bounty during unitaryHACK.

Tasks

• Create a new set of unit tests (potentially in test_viz.py) that covers all possible configuration options of the functions on the viz module. To see the current coverage of the module check codecov.

Explanation

The viz utility module contains helper functions to draw various graphs and generate plots using Matplotlib. This module provides several ways to personalize and configure the resulting figures via the use of function parameters.

The complete set of possible configurations has not been added to the FlamingPy test suite.

Requirements

• Instead of using image comparison to evaluate the output of the functions of the module I would instead recommend using mocking to mock matplotlib's methods and check that appropriate calls with appropriate arguments are passed to those (mocked) methods.
• The implementation of this test should cover >99% of the viz module.

[nariman87]

We can archive this till we work on different backends or next HACK competitions.