Issue #22002: Make full use of test discovery in test sub-packages.

Adds `load_package_tests` function to test.support, uses it in test_asyncio,
test_email, test_json, test_tools, test_importlib and all test_importlib
sub-packages to implement test discovery.

Lib/test/test_asyncio/__init__.py

@@ -1,29 +1,10 @@
 import os
-import sys
-import unittest
-from test.support import run_unittest, import_module
+from test.support import load_package_tests, import_module
 
 # Skip tests if we don't have threading.
 import_module('threading')
 # Skip tests if we don't have concurrent.futures.
 import_module('concurrent.futures')
 
-
-def suite():
-    tests = unittest.TestSuite()
-    loader = unittest.TestLoader()
-    for fn in os.listdir(os.path.dirname(__file__)):
-        if fn.startswith("test") and fn.endswith(".py"):
-            mod_name = 'test.test_asyncio.' + fn[:-3]
-            try:
-                __import__(mod_name)
-            except unittest.SkipTest:
-                pass
-            else:
-                mod = sys.modules[mod_name]
-                tests.addTests(loader.loadTestsFromModule(mod))
-    return tests
-
-
-def test_main():
-    run_unittest(suite())
+def load_tests(*args):
+    return load_package_tests(os.path.dirname(__file__), *args)

Lib/test/test_asyncio/__main__.py

@@ -1,5 +1,4 @@
-from . import test_main
+from . import load_tests
+import unittest
 
-
-if __name__ == '__main__':
-    test_main()
+unittest.main()