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

[andersk]

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

Example:

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.

$ 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.)

$ 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

[rust-highfive]

r? @dtolnay

(rust-highfive has picked a reviewer for you, use r? to override)

[dtolnay]

Thanks! This is great.

[dtolnay]

@bors r+

[bors]

📌 Commit 57c72ab has been approved by dtolnay

[jonas-schievink]

failed in #81385 (comment)

@bors r- rollup=iffy

[andersk]

thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', library/test/src/lib.rs:347:75

I guess that could happen if a test returns after timing out. Will fix.

[dtolnay]

@bors r+

[bors]

📌 Commit b05788e has been approved by dtolnay

[bors]

⌛ Testing commit b05788e with merge 1483e67...

[bors]

☀️ Test successful - checks-actions
Approved by: dtolnay
Pushing 1483e67 to master...