bpo-45229: Make datetime tests discoverable (GH-28615)

(cherry picked from commit d441437ee7)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

Lib/test/test_datetime.py

@@ -1,12 +1,12 @@
 import unittest
 import sys
 
-from test.support import run_unittest
 from test.support.import_helper import import_fresh_module
 
 
 TESTS = 'test.datetimetester'
 
+def load_tests(loader, tests, pattern):
     try:
         pure_tests = import_fresh_module(TESTS, fresh=['datetime', '_strptime'],
                                         blocked=['_datetime'])
@@ -17,13 +17,12 @@ finally:
         # XXX: but it does not, so we have to cleanup ourselves.
         for modname in ['datetime', '_datetime', '_strptime']:
             sys.modules.pop(modname, None)
+
     test_modules = [pure_tests, fast_tests]
     test_suffixes = ["_Pure", "_Fast"]
     # XXX(gb) First run all the _Pure tests, then all the _Fast tests.  You might
     # not believe this, but in spite of all the sys.modules trickery running a _Pure
     # test last will leave a mix of pure and native datetime stuff lying around.
-all_test_classes = []
-
     for module, suffix in zip(test_modules, test_suffixes):
         test_classes = []
         for name, cls in module.__dict__.items():
@@ -50,10 +49,9 @@ for module, suffix in zip(test_modules, test_suffixes):
                 sys.modules.update(cls_._save_sys_modules)
             cls.setUpClass = setUpClass
             cls.tearDownClass = tearDownClass
-    all_test_classes.extend(test_classes)
+            tests.addTests(loader.loadTestsFromTestCase(cls))
+    return tests
 
-def test_main():
-    run_unittest(*all_test_classes)
 
 if __name__ == "__main__":
-    test_main()
+    unittest.main()