Tests: move test_posix.py and test_os.py into a new test_os package

[vstinner]

In total, test_posix.py and test_os.py are the second largest test in the Python test suite: 8,193 lines.

I propose to:

• (1) merge these two files into a new single test_os package
• and (2) split these files into smaller files to ease their maintenance.

Linked PRs

• gh-139322: Create test_os package #139323
• gh-139322: Create test_os package #139453
• gh-139322: Remove redundant test_os.Win32ErrorTests #139477
• gh-139322: Create test_os.test_process #139480
• gh-139322: Reenable test_os.test_getlogin() #139498

[serhiy-storchaka]

test_os and test_posix contain tests for the same functions (except pure Python functions added in os). It is often not easy to find an appropriate place for new test, because there are tests in both modules.

Until recently (see #111912), test_posix was ran only on Posix.