add test that multi-threaded panics aren't interleaved

tests/ui/backtrace/synchronized-panic-handler.rs

@@ -0,0 +1,15 @@
+//@ run-pass
+//@ check-run-results
+//@ edition:2021
+use std::thread;
+const PANIC_MESSAGE: &str = "oops oh no woe is me";
+
+fn entry() {
+    panic!("{PANIC_MESSAGE}")
+}
+
+fn main() {
+    let (a, b) = (thread::spawn(entry), thread::spawn(entry));
+    assert_eq!(&**a.join().unwrap_err().downcast::<String>().unwrap(), PANIC_MESSAGE);
+    assert_eq!(&**b.join().unwrap_err().downcast::<String>().unwrap(), PANIC_MESSAGE);
+}

tests/ui/backtrace/synchronized-panic-handler.run.stderr

@@ -0,0 +1,5 @@
+thread '<unnamed>' panicked at $DIR/synchronized-panic-handler.rs:thread '8<unnamed>:5' panicked at :
+oops oh no woe is me$DIR/synchronized-panic-handler.rs
+:note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
+8:5:
+oops oh no woe is me