[3.11] Use unittest to collect and run distutils tests

[serhiy-storchaka]

Currently every test module in the distutils.tests package has function test_suite() which explicitly creates unittest.TestLoader and loads it by calling loadTestsFromTestCase() for every test class. Then test_main merges all these tests and runs them with test.support.run_unittest(). But test.support.run_unittest() will be removed (see #111165). Also, the distutils.tests package cannot be run via unittest, it does not search tests in submodules by default, and when run tests in submodules separately via unittest, it misses some doctests because they cannot be automatically loaded.

The following PR contains the following changes:

• 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

Linked PRs

• [3.11] gh-111309: Use unittest to collect and run distutils tests #111311