tests: use needs-threads instead of ignore-emscripten

tests/ui/array-slice-vec/box-of-array-of-drop-1.rs

@@ -1,12 +1,12 @@
 //@ run-pass
 //@ needs-unwind
+//@ needs-threads
+
 #![allow(overflowing_literals)]
 
 // Test that we cleanup a fixed size Box<[D; k]> properly when D has a
 // destructor.
 
-//@ ignore-emscripten no threads support
-
 use std::thread;
 use std::sync::atomic::{AtomicUsize, Ordering};