Fix test warnings

[gretchenfrage]

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.)