gh-139322: Create test_os.test_process

[vstinner]

• Issue: Tests: move test_posix.py and test_os.py into a new test_os package #139322

[vstinner]

@serhiy-storchaka wrote:

I think splitting tests into separate files for each function is too much. We have separate classes for grouping.

I created this PR to see if it would be acceptable to move some test cases "per function" anyway, or not.

[github-actions]

This PR is stale because it has been open for 30 days with no activity.