[3.11] gh-111309: Use unittest to collect and run distutils tests (GH-111311)

* 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