test/test.rs instructions run into serde compat issues

Right now, the test/test.rs instructions say to use nightly-2023-09-04. But it appears that serde-1.0.198 uses features which this version of nightly considers to be forbidden:

error[E0658]: use of unstable library feature 'saturating_int_impl'
   --> /home/nickm/.cargo/registry/src/index.crates.io-6f17d22bba15001f/serde-1.0.198/src/lib.rs:279:13
    |
279 |     pub use self::core::num::Saturating;
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: see issue #87920 <https://github.com/rust-lang/rust/issues/87920> for more information
    = help: add `#![feature(saturating_int_impl)]` to the crate attributes to enable

(There are over 200 errors like this.)

The likeliest solutions seem to be "downgrade serde in cargo.lock" or "update the version of nightly we use for tests."

I'm going to make an MR to do the latter.