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

[miss-islington]

(cherry picked from commit 1c7ed7e)

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

• Issue: Make pdb completion work with alias, convenience variables etc. #110944

[bedevere-bot]

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