Add unit tests for vis/_vis_utils

[FedeMPouzols]

This adds unit tests for vis/_vis_utils. So far in tests/unit/vis we only had a few unit tests for optimised_functions.py This PR adds a set of unit tests (230+) to cover the code under src/xradio/vis/_vis_utils. It adds a conftest with pytest fixtures, and an on-the-fly test MS geneator used in these tests. It also includes a few fixes for corner cases in the _tables read/write functions.

All pytests tests runtime before: ~55s, after: 1m25s Coverage (all repo) before: 65%, after: 87%

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This contribution is made under the current ALMA software agreements.
(c) European Southern Observatory, 2024
Copyright by ESO (in the framework of the ALMA collaboration)

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