gh-121160: Add a test for readline.set_history_length

[encukou]

I could not reproduce the reported issue, but, I didn't try very hard to test on various readline libraries. Regardless, set_history_length was missing a test. This PR should show if the error exists on CPython's buildbot platforms. (And if it doesn't and it's merged, it should fail for redistributors with who build with different libraries.)

My copy of readline (Fedora 40) reports an extra history element after reading the file; IMO that's not the end of the world.

• Issue: gh-121160

[bedevere-bot]

:robot: New build scheduled with the buildbot fleet by @encukou for commit ba60eae46b4f925a72925da638ea0b2b9f3edd85 :robot:

If you want to schedule another build, you need to add the :hammer: test-with-buildbots label again.

[bedevere-bot]

:robot: New build scheduled with the buildbot fleet by @encukou for commit 626439e77f854f1eb8016532f0e5d86b2fb7cd35 :robot:

If you want to schedule another build, you need to add the :hammer: test-with-buildbots label again.

[miss-islington-app[bot]]

Thanks @encukou for the PR 🌮🎉.. I'm working now to backport this PR to: 3.13. 🐍🍒⛏🤖

[miss-islington-app[bot]]

Thanks @encukou for the PR 🌮🎉.. I'm working now to backport this PR to: 3.12. 🐍🍒⛏🤖

[bedevere-app[bot]]

GH-121856 is a backport of this pull request to the 3.13 branch.

[bedevere-app[bot]]

GH-121857 is a backport of this pull request to the 3.12 branch.