[3.12] gh-110944: Move pty helper to test.support and add basic pdb completion test (GH-111826) (GH-112024)

gh-110944: Move pty helper to test.support and add basic pdb completion test (GH-111826)
(cherry picked from commit 1c7ed7e9ebc53290c831d7b610219fa737153a1b)

Co-authored-by: Tian Gao <gaogaotiantian@hotmail.com>