enable test_join test in Miri

Author: RalfJung

library/alloc/tests/vec.rs

@@ -1906,6 +1906,7 @@ fn test_stable_pointers() {
     assert_eq!(*v0, 13);
 
     // Smoke test that would fire even outside Miri if an actual relocation happened.
+    // Also ensures the pointer is still writeable after all this.
     *v0 -= 13;
     assert_eq!(v[0], 0);
 }

library/core/tests/future.rs

@@ -30,7 +30,6 @@ fn poll_n(val: usize, num: usize) -> PollN {
 }
 
 #[test]
-#[cfg_attr(miri, ignore)] // self-referential generators do not work with Miri's aliasing checks
 fn test_join() {
     block_on(async move {
         let x = join!(async { 0 }).await;