fallout: part of changes to compile-fail tests. (follows same pattern as prior two commits.)

src/test/compile-fail/autoderef-full-lval.rs

@@ -8,6 +8,9 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![allow(unknown_features)]
+#![feature(box_syntax)]
+
 struct clam {
     x: Box<isize>,
     y: Box<isize>,

src/test/compile-fail/borrow-tuple-fields.rs

@@ -8,6 +8,9 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![allow(unknown_features)]
+#![feature(box_syntax)]
+
 struct Foo(Box<int>, int);
 
 struct Bar(int, int);

src/test/compile-fail/borrowck-array-double-move.rs

@@ -8,6 +8,9 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![allow(unknown_features)]
+#![feature(box_syntax)]
+
 fn f() {
     let mut a = [box 0i, box 1i];
     drop(a[0]);