Add missing __init__.py for tests/post_training packages

[ecnal-cienet]

Problem

tests/post_training/ and tests/post_training/integration/ were missing
__init__.py, causing pylint to fail with:

E0611: No name 'grpo_trainer_correctness_test' in module 'tests.integration' (no-name-in-module)

This was introduced in #3427 which added files under these directories but
did not register them as Python packages.

Fix

Add __init__.py with the standard license header and pathwaysutils.initialize() to both directories, consistent with other test packages in this repo.

Checklist

Before submitting this PR, please make sure (put X in square brackets):

• I have performed a self-review of my code. For an optional AI review, add the gemini-review label.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run end-to-end tests tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed, including adding new documentation pages to the relevant Table of Contents (toctree directive) as explained in our documentation.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!