Remove redundant imports (#12817)

# Objective

- There are several redundant imports in the tests and examples that are
not caught by CI because additional flags need to be passed.

## Solution

- Run `cargo check --workspace --tests` and `cargo check --workspace
--examples`, then fix all warnings.
- Add `test-check` to CI, which will be run in the check-compiles job.
This should catch future warnings for tests. Examples are already
checked, but I'm not yet sure why they weren't caught.

## Discussion

- Should the `--tests` and `--examples` flags be added to CI, so this is
caught in the future?
- If so, #12818 will need to be merged first. It was also a warning
raised by checking the examples, but I chose to split off into a
separate PR.

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Co-authored-by: François Mockers <francois.mockers@vleue.com>