bpo-45229: Make tkinter tests discoverable (GH-28637) (GH-28927)

(cherry picked from commit f59ed3c310a7ceebf2a56a84ea969a7f75d95b64)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
diff --git a/Lib/test/test_tk.py b/Lib/test/test_tk.py
index 59842a5..69cc232 100644
--- a/Lib/test/test_tk.py
+++ b/Lib/test/test_tk.py
@@ -6,11 +6,9 @@
 # Skip test if tk cannot be initialized.
 support.requires('gui')
 
-from tkinter.test import runtktests
+def load_tests(loader, tests, pattern):
+    return loader.discover('tkinter.test.test_tkinter')
 
-def test_main():
-    support.run_unittest(
-            *runtktests.get_tests(text=False, packages=['test_tkinter']))
 
 if __name__ == '__main__':
-    test_main()
+    unittest.main()