Fix test-engine-core unit tests and cleanup module

[timlrx]

Description

• Fix failed unit tests in /test-engine-core/io (Thanks @imda-kelvinkok!)
  • Remove pytest.skip for the test cases
• Misc housekeeping as discussed
  • Remove test artifacts from git
  • Flatten dir structure of io tests and move all test files into a single user_defined_files folder
• Publish build files in dist/ for interim development purposes

How to Test

pytest test-engine-core/tests/io/ -p no:warnings

Checklist

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• [x] I have added appropriate unit tests or functional tests for the changes made.
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• [x] I have rebased my branch onto the latest commit of the main branch.
• [x] I have squashed or reorganized my commits into logical units.
• [x] I have added any necessary dependencies or packages to the project's build configuration.
• [x] I have performed a self-review of my own code.
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