Add a test for overshadowing redirects

[InSyncWithFoo]

Summary

Resolves #15257.

Test Plan

cargo nextest run and cargo insta test.

[InSyncWithFoo]

The test added currently does not pass, but it will once #15258 is merged. For the record, here's what the message looks like:

────────────
     Summary [  10.999s] 3795 tests run: 3794 passed, 1 failed, 23 skipped
        FAIL [   0.004s] ruff_linter rule_redirects::tests::overshadowing_redirects
──── STDOUT:             ruff_linter rule_redirects::tests::overshadowing_redirects

running 1 test
test rule_redirects::tests::overshadowing_redirects ... FAILED

failures:

failures:
    rule_redirects::tests::overshadowing_redirects

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 2217 filtered out; finished in 0.00s

──── STDERR:             ruff_linter rule_redirects::tests::overshadowing_redirects
thread 'rule_redirects::tests::overshadowing_redirects' panicked at crates/ruff_linter/src/rule_redirects.rs:155:17:
Rule RUF025 is overshadowed by a redirect, which points to C420.

As for Clippy, I'm not sure why it says the imports are unused.

[MichaReiser]

Thank you so much for fixing this so quickly!

Clippy complaint because the mod test missed the #[cfg(test)] gating.

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.