Allow to rustfix unused_variables lint.

[cjgillot]

What does this PR try to resolve?

The rest cases cache_messages::wacky_hashless_fingerprint, warning_override::config, warning_override::requires_nightly, warning_override::rustc_caching_deny_first and warning_override::rustc_caching_allow_first are too strict and prevent merging rust-lang/rust#142390.

This PR relaxes the expected output to allow for the cargo fix suggestion.

How to test and review this PR?

Checkout rustc at rust-lang/rust#142390, change src/tools/cargo to this branch, and verify that cargo tests pass.

[rustbot]

r? @weihanglo

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[weihanglo]

Thanks!

---

https://triage.rust-lang.org/gha-logs/rust-lang/rust/50615623578

For porserity, the failure looks like

---- expected: tests/testsuite/warning_override.rs:44:27
++++ actual:   stderr
   1    1 | [WARNING] unused variable: `x`
   2    2 |  --> src/main.rs:1:17
   3    3 |   |
   4    4 | 1 | fn main() { let x = 3; }
   5    5 |   |                 ^ [HELP] if this is intentional, prefix it with an underscore: `_x`
   6    6 |   |
   7    7 |   = [NOTE] `#[warn(unused_variables)]` [..]on by default
   8    8 | 
   9      - [WARNING] `foo` (bin "foo") generated 1 warning
        9 + [WARNING] `foo` (bin "foo") generated 1 warning (run `cargo fix --bin "foo"` to apply 1 suggestion)
  10   10 | [FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
  11   11 | [ERROR] warnings are denied by `build.warnings` configuration