enable clippy for tests

[ririsoft]

This PR enables clippy checks for tests and examples during CI.

Description

Clippy is valuable for tests and examples too, it can spot some mistakes that would make a test wrong.

Motivation and Context

Having zero Clippy errors after the first clone/fork improves the contributor experience.

How Has This Been Tested?

cargo test --all --features unstable
cargo clippy --tests --examples

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[jbr]

👍

[jbr]

Thanks!