[3.11] gh-111309: Use unittest to collect and run distutils tests

[serhiy-storchaka]

• use unittest.main() instead of run_unittest(test_suite()) to run tests from modules via the CLI
• add explicit load_tests() to load doctests
• use test.support.load_package_tests() to load tests in submodules of distutils.tests
• removes no longer needed test_suite() functions

• Issue: [3.11] Use unittest to collect and run distutils tests #111309