Activate clippy for `test` target

[vobst]

Currently, our automated code style checking, i.e., make codestyle-check, does not include the test target. This means that there are a lot of warnings if you run cargo clippy --tests -- -D clippy:all.

Fix them and change cargo clippy -- -D clippy::all -D missing_docs to cargo clippy --all-targets -- -D clippy::all -D missing_docs in the Makefile.