Fix test warnings

On the master branch, there are 18 warnings which occur only in test modules. These warnings are not logged by `cargo check` alone, but they are logged if you run `cargo check --tests`. This PR: - Fixes those warnings. - Makes the CI run `cargo check --tests` to catch such things again in the future. (Note: `cargo check --tests` _also_ logs warnings in the normal code too.) (Note: I noticed this because I'm using the Zed editor which does this automatically in an integrated way.)