Auto merge of #81367 - andersk:join-test-threads, r=dtolnay

libtest: Wait for test threads to exit after they report completion

Otherwise we can miss bugs where a test reports that it succeeded but then panics within a TLS destructor.

Example:

```rust
use std::thread::sleep;
use std::time::Duration;

struct Foo;

impl Drop for Foo {
    fn drop(&mut self) {
        sleep(Duration::from_secs(1));
        panic!()
    }
}

thread_local!(static FOO: Foo = Foo);

#[test]
pub fn test() {
    FOO.with(|_| {});
}
```

Before this fix, `cargo test` incorrectly reports success.

```console
$ cargo test
    Finished test [unoptimized + debuginfo] target(s) in 0.01s
     Running target/debug/deps/panicking_test-85130fa46b54f758

running 1 test
test test ... ok

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

$ echo $?
0
```

After this fix, the failure is visible. (The entire process is aborted due to #24479.)

```console
$ cargo test
    Finished test [unoptimized + debuginfo] target(s) in 0.01s
     Running target/debug/deps/panicking_test-76180625bc2ee3c9

running 1 test
thread 'test' panicked at 'explicit panic', src/main.rs:9:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
fatal runtime error: failed to initiate panic, error 5
error: test failed, to rerun pass '--bin panicking-test'

Caused by:
  process didn't exit successfully: `/tmp/panicking-test/target/debug/deps/panicking_test-76180625bc2ee3c9 --nocapture` (signal: 6, SIGABRT: process abort signal)

$ echo $?
101
```

0 files changed, 0 insertions, 0 deletions