message: Add an explanatory comment to the test.
diff: @@ -10,6 +10,7 @@ use std::thread;
  use std::{mem, ptr};
  extern "C" fn thread_start(_null: *mut libc::c_void) -> *mut libc::c_void {
  + // Yield the thread several times so that other threads can join it.
  thread::yield_now();
  thread::yield_now();
  ptr::null_mut()
  