Issue #25220: Fix "-m test --forever"

* Fix "-m test --forever": replace _test_forever() with self._test_forever()
* Add unit test for --forever
* Add unit test for a failing test
* Fix also some pyflakes warnings in libregrtest
diff --git a/Lib/test/libregrtest/main.py b/Lib/test/libregrtest/main.py
index fdb925d..e1a99fb 100644
--- a/Lib/test/libregrtest/main.py
+++ b/Lib/test/libregrtest/main.py
@@ -319,7 +319,7 @@
 
     def run_tests(self):
         if self.ns.forever:
-            self.tests = _test_forever(list(self.selected))
+            self.tests = self._test_forever(list(self.selected))
             self.test_count = ''
             self.test_count_width = 3
         else: