cargo: Suggest `--nocapture` flag when a test fails with abort

Problem

Consider the following test

#[test]
fn rayon_panic() {
    rayon::spawn(|| panic!("o_O"));
    loop {}
}

cargo test for it produces the following output:

λ cargo t
   Compiling rt v0.1.0 (/home/matklad/tmp/rt)
    Finished test [unoptimized + debuginfo] target(s) in 0.17s
     Running unittests src/lib.rs (target/debug/deps/rt-fb003f4fc4a43299)

running 1 test
error: test failed, to rerun pass '--lib'

Caused by:
  process didn't exit successfully: `/home/matklad/tmp/rt/target/debug/deps/rt-fb003f4fc4a43299` (signal: 6, SIGABRT: process abort signal)

Note how it’s super unclear what exactly failed and why. Passing --nocapture flags helps a bit, but only an expert user would realize that such a flag might be helpful!

λ cargo t -- --nocapture
    Finished test [unoptimized + debuginfo] target(s) in 0.01s
     Running unittests src/lib.rs (target/debug/deps/rt-fb003f4fc4a43299)

running 1 test
thread '<unnamed>' panicked at 'o_O', src/lib.rs:3:21
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Rayon: detected unexpected panic; aborting
error: test failed, to rerun pass '--lib'

Caused by:
  process didn't exit successfully: `/home/matklad/tmp/rt/target/debug/deps/rt-fb003f4fc4a43299 --nocapture` (signal: 6, SIGABRT: process abort signal)

---

What happens here is that the test calls std::process::abort. Because, by default, cargo test captures stderr, and because abort doesn’t unwind, libtest doesn’t get a chance to flush stderr. This seems like a fundamental limitation of libtest.

Proposed Solution

It’d be cool if cargo realised that cargo test process was killed with a singnal, and suggested re-running with --nocapture in this case. Note that this won’t always help (it might be the case that a test died without printing anything to stderr), but it doesn’t seem that the note like the following would hurt:

error: test failed, to rerun pass '--lib'
 note: test was terminated by the signal, stderr might be truncated, pass `--nocapture` disable output buffering.

Notes

No response