Cleanups attributes in tests

[samueltardieu]

Cleanup existing tests:

• remove unneeeded attributes (such as #![allow(unused)])
• use expect instead of allow when possible (in the absence of a .fixed file, or if the lint still triggers in the .fixed file, and remove unnecessary allow attributes

changelog: none

[rustbot]

r? @Jarcho

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The reviewer was selected based on:

• Owners of files modified in this PR: 7 candidates
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[samueltardieu]

Since we talked about this yesterday at the AllHands:
r? ada4a

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.