[3.14] gh-146059: Cleanup pickle fast_save_enter() test (GH-146481) (#146509)

gh-146059: Cleanup pickle fast_save_enter() test (GH-146481)

Remove {"key": data}, it's not required to reproduce the bug.
Simplify also deep_nested_struct(): remove the seed parameter.
Fix a typo in a comment.
(cherry picked from commit 0c7a75aeef4dae87f02536ed4c42a57c13ef20e2)

Co-authored-by: Victor Stinner <vstinner@python.org>