Add async-io tests back to CI

[notgull]

Closes #145

[taiki-e]

CI failures are:

• A regression of the espidf support introduced in the recent nightly (by std::net::bind using -1 for openbsd which in turn sets it to somaxconn. rust-lang/rust#119026).
  https://github.com/smol-rs/polling/actions/runs/7429401335/job/20217927199?pr=172
• New warning with buggy suggestion dead_code suggestion for unused fields does not account for changes to auto trait implementations rust-lang/rust#119645
  https://github.com/smol-rs/polling/actions/runs/7429401335/job/20217927719?pr=172
• Duplicate [patch.crates-io] table (the current async-io already have it) -- this is a bug of this PR.
  https://github.com/smol-rs/polling/actions/runs/7429401335/job/20217927400?pr=172

[notgull]

Thank you for the insight @taiki-e ! I can take care of the latter and disable the two former

[taiki-e]

(CI failures unrelated to this PR have been fixed in #173)