Add allow(unstable) to more tests

Author: brson

src/test/run-pass/ifmt.rs

@@ -14,6 +14,7 @@
 #![deny(warnings)]
 #![allow(unused_must_use)]
 #![allow(unknown_features)]
+#![allow(unstable)]
 #![feature(box_syntax)]
 
 use std::fmt;

src/test/run-pass/issue-11958.rs

@@ -12,6 +12,7 @@
 
 // Pretty printing tests complain about `use std::predule::*`
 #![allow(unused_imports)]
+#![allow(unstable)]
 
 // We shouldn't need to rebind a moved upvar as mut if it's already
 // marked as mut

src/test/run-pass/issue-16671.rs

@@ -14,6 +14,7 @@
 
 // Pretty printing tests complain about `use std::predule::*`
 #![allow(unused_imports)]
+#![allow(unstable)]
 
 // A var moved into a proc, that has a mutable loan path should
 // not trigger a misleading unused_mut warning.