tests(fix): Exclude unused features from failing a test

[polarathene]

This resolves a current test failure (CI failure log) caused by lint warnings emitted to stderr due to (introduced from the recent Nightly version bump):

• A new lint unused_features was introduced in a March 2026 Nightly release (and later part of Rust 1.96): Lint unused features rust-lang/rust#152164
• A follow-up in March 2026 adjusted the scope of the lint to unstable features: Only lint unused features if they are unstable rust-lang/rust#153610

I am not sure how you'd like to approach resolving this.

• I chose to only apply it to the affected test case rather than globally within c-scape/src/lib.rs by adding a #![allow(unused_features)].
• Another alternative would be to expect the stderr output instead with the lint warnings.

CI test failure output (click to expand)

CI failure log, relevant snippet:

     Running tests/example_crates.rs (target/debug/deps/example_crates-4423017ae3c0b04a)

running 12 tests
test example_crate_c_gull_example_panic_abort ... ok
test example_crate_c_gull_unwinding ... ok
test example_crate_c_gull_example ... ok
test example_crate_c_scape_example ... ok
test example_crate_c_scape_example_panic_abort ... ok
test example_crate_c_scape_unwinding ... ok
test example_crate_c_gull_lto ... ok
test example_crate_custom_allocator ... ok
test example_crate_libc_replacement ... FAILED
test example_crate_dns ... ok
test example_crate_threadsafe_setenv_lookup ... ok
test example_crate_threadsafe_setenv_getenv ... ok

failures:

---- example_crate_libc_replacement stdout ----

thread 'example_crate_libc_replacement' (9924) panicked at tests/example_crates.rs:39:40:
Unexpected stderr, failed diff original var
├── original: 
├── diff: 
│   --- 	orig
│   +++ 	var
│   @@ -0,0 +1,14 @@
│   +warning: feature `thread_local` is declared but not used
│   + --> /home/runner/work/c-ward/c-ward/c-scape/src/lib.rs:5:12
│   +  |
│   +5 | #![feature(thread_local)] // for `pthread_getspecific` etc.
│   +  |            ^^^^^^^^^^^^
│   +  |
│   +  = note: `#[warn(unused_features)]` (part of `#[warn(unused)]`) on by default
│   +
│   +warning: feature `linkage` is declared but not used
│   + --> /home/runner/work/c-ward/c-ward/c-scape/src/lib.rs:8:12
│   +  |
│   +8 | #![feature(linkage)] // for `malloc` etc.
│   +  |            ^^^^^^^
│   +
└── var as str: warning: feature `thread_local` is declared but not used
     --> /home/runner/work/c-ward/c-ward/c-scape/src/lib.rs:5:12
      |
    5 | #![feature(thread_local)] // for `pthread_getspecific` etc.
      |            ^^^^^^^^^^^^
      |
      = note: `#[warn(unused_features)]` (part of `#[warn(unused)]`) on by default
    
    warning: feature `linkage` is declared but not used
     --> /home/runner/work/c-ward/c-ward/c-scape/src/lib.rs:8:12
      |
    8 | #![feature(linkage)] // for `malloc` etc.
      |            ^^^^^^^
    

command=`cd "example-crates/libc-replacement" && "cargo" "run" "--quiet" "--target=x86_64-unknown-linux-gnu"`
code=0
stdout=```
Hello, world! uid=1001
Hello world with printf! gid=1001
Hello world from `atexit_func`
```

stderr=```
warning: feature `thread_local` is declared but not used
 --> /home/runner/work/c-ward/c-ward/c-scape/src/lib.rs:5:12
  |
5 | #![feature(thread_local)] // for `pthread_getspecific` etc.
  |            ^^^^^^^^^^^^
  |
  = note: `#[warn(unused_features)]` (part of `#[warn(unused)]`) on by default

warning: feature `linkage` is declared but not used
 --> /home/runner/work/c-ward/c-ward/c-scape/src/lib.rs:8:12
  |
8 | #![feature(linkage)] // for `malloc` etc.
  |            ^^^^^^^

```

---

Separately within this same file is a removal that was missed as part of PR #169, which didn't cause any failures but does removal a warning (as per referenced PR).