Add #![allow(stable_features)] to ease review.

That is, opting to ignore stable `#![feature(..)]`, under my theory
that a reviewer would prefer as much stuff to be bucketed under "ah
another `#![allow(..)]` here." as possible.

src/test/ui/run-pass/consts/const-fn.rs

@@ -9,6 +9,8 @@
 // except according to those terms.
 
 // run-pass
+#![allow(stable_features)]
+
 // A very basic test of const fn functionality.
 
 #![feature(const_fn, const_indexing)]

src/test/ui/run-pass/issues/issue-29914.rs

@@ -9,6 +9,7 @@
 // except according to those terms.
 
 // run-pass
+#![allow(stable_features)]
 
 #![feature(const_indexing)]
 

src/test/ui/run-pass/iterators/iter-cloned-type-inference.rs

@@ -9,6 +9,8 @@
 // except according to those terms.
 
 // run-pass
+#![allow(stable_features)]
+
 // Test to see that the element type of .cloned() can be inferred
 // properly. Previously this would fail to deduce the type of `sum`.
 

src/test/ui/run-pass/panics/panic-handler-set-twice.rs

@@ -9,6 +9,7 @@
 // except according to those terms.
 
 // run-pass
+#![allow(stable_features)]
 
 #![feature(std_panic)]