reduce size of test_from_iter_specialization_with_iterator_adapters test in Miri

Author: RalfJung

library/alloc/tests/vec.rs

@@ -894,7 +894,7 @@ fn test_from_iter_partially_drained_in_place_specialization() {
 #[test]
 fn test_from_iter_specialization_with_iterator_adapters() {
     fn assert_in_place_trait<T: InPlaceIterable>(_: &T) {};
-    let src: Vec<usize> = vec![0usize; 65535];
+    let src: Vec<usize> = vec![0usize; if cfg!(miri) { 256 } else { 65535 }];
     let srcptr = src.as_ptr();
     let iter = src
         .into_iter()