Running `cargo miri test` fails due to thread outliving main thread

With the minimal example:

```
#[cfg(test)]
pub mod tests {

#[test]
fn mf_test_delay() {
block_on(Delay::new(Duration::from_millis(500)));
}
}

```
running the command:
```
MIRIFLAGS="-Zmiri-disable-isolation" cargo miri test mf_
```
will cause the tests to pass, but the overall command will fail with the following output:

```
❯ MIRIFLAGS="-Zmiri-disable-isolation" cargo miri test mf_
Finished `test` profile [unoptimized + debuginfo] target(s) in 0.16s
Running unittests src/lib.rs ()

running 1 tests
test stream::futures_unordered::mapped_futures::tests::mf_test_delay ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 1.68s

error: the main thread terminated without waiting for all remaining threads

note: set `MIRIFLAGS=-Zmiri-ignore-leaks` to disable this check

error: aborting due to 1 previous error

error: test failed, to rerun pass `--lib`
```

Rust version:
❯ rustc --version
rustc 1.85.0-nightly (0aeaa5eb2 2024-12-14)

Reproduce the issue with `MIRIFLAGS="-Zmiri-disable-isolation" cargo miri test mf_` and inspect `stream::futures_unordered::mapped_futures::tests::mf_test_delay`, using the minimal `Delay::new` example as the starting point. Trace which remaining thread causes Miri's leak check after the test passes; done means the command completes without the thread-outliving-main error.

Written by the indexing model from the issue text.