reduce duplication of rock image testing code across our repositories

[ca-scribner]

Why it needs to get done

Our reusable rock workflows execute tests to assert our images perform as expected. These are defined in a tox.ini that is put in every rock subdirectory (mlserver-sklearn, mlserver-xgboost). Some of these tests also use near-duplicate python scripts (mlserver-sklearn, mlserver-xgboost).

These files are not easy to read and also largely duplicated - we should think on a more maintainable way to test these rocks

What needs to get done

Research needed to decide the best way forward. Maybe we can refactor the tox.ini file so there's just one per repo? This may take some effort to do for the integration test environment because that env pulls and runs the integration tests of the charm that uses these images.

wrt test_rock.py, that file is nearly duplicated - the only thing that changes in that file is which files we look for in the rock. This could be replaced by a generic script (kept centrally, like in chisme or elsewhere) that accepted a --file path/to/file/in/rock argument

When is the task considered done

Reduced duplication, ideally removing the copy/pasted tox.ini and test_rock.py files

[syncronize-issues-to-jira]

Thank you for reporting us your feedback!

The internal ticket has been created: https://warthogs.atlassian.net/browse/KF-5121.

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[orfeas-k]

This could be tackled in parallel with #785.

[ca-scribner]

This is similar to #817