pythongh-121651: Fix pdb header test (python#121724)

Author: gaogaotiantian

Lib/test/test_pdb.py

@@ -3389,7 +3389,12 @@ def test_header(self):
         header = 'Nobody expects... blah, blah, blah'
         with ExitStack() as resources:
             resources.enter_context(patch('sys.stdout', stdout))
+            # patch pdb.Pdb.set_trace() to avoid entering the debugger
             resources.enter_context(patch.object(pdb.Pdb, 'set_trace'))
+            # We need to manually clear pdb.Pdb._last_pdb_instance so a
+            # new instance with stdout redirected could be created when
+            # pdb.set_trace() is called.
+            pdb.Pdb._last_pdb_instance = None
             pdb.set_trace(header=header)
         self.assertEqual(stdout.getvalue(), header + '\n')