gh-121651: Fix pdb header test

[gaogaotiantian]

In the header test for pdb.set_trace, we redirected stdout to a string buffer, but that would only work when we create the Pdb instance after the patch. Due to the changes in #121451 , that's not always guaranteed. So for this test, we clear the Pdb._last_pdb_instance before pdb.set_trace(), which enforces pdb.set_trace() to create a new Pdb instance which has the redirected stdout.

• Issue: test_pdb fails with an --forever argument #121651

[gaogaotiantian]

@Eclips4 feel free to take a look at this fix and confirm this works. I tried it locally and ./python -m test test_pdb --forever worked fine.

[Eclips4]

@Eclips4 feel free to take a look at this fix and confirm this works. I tried it locally and ./python -m test test_pdb --forever worked fine.

Yes, I can confirm that this fixes the problem. Thanks!