Auto merge of #78227 - SergioBenitez:test-stdout-threading, r=m-ou-se

Capture output from threads spawned in tests

This is revival of #75172.

Original text:
> Fixes #42474.
>
> r? `@​dtolnay` since you expressed interest in this, but feel free to redirect if you aren't the right person anymore.

---

Closes #75172.

compiler/rustc_interface/src/util.rs

@@ -115,6 +115,11 @@ impl Write for Sink {
         Ok(())
     }
 }
+impl io::LocalOutput for Sink {
+    fn clone_box(&self) -> Box<dyn io::LocalOutput> {
+        Box::new(Self(self.0.clone()))
+    }
+}
 
 /// Like a `thread::Builder::spawn` followed by a `join()`, but avoids the need
 /// for `'static` bounds.