Merge set_panic and set_print into set_output_capture.

There were no use cases for setting them separately.
Merging them simplifies some things.

src/test/ui/panic-while-printing.rs

@@ -1,11 +1,11 @@
 // run-pass
 // ignore-emscripten no subprocess support
 
-#![feature(set_stdio)]
+#![feature(internal_output_capture)]
 
 use std::fmt;
 use std::fmt::{Display, Formatter};
-use std::io::set_panic;
+use std::io::set_output_capture;
 use std::sync::{Arc, Mutex};
 
 pub struct A;
@@ -17,7 +17,7 @@ impl Display for A {
 }
 
 fn main() {
-    set_panic(Some(Arc::new(Mutex::new(Vec::new()))));
+    set_output_capture(Some(Arc::new(Mutex::new(Vec::new()))));
     assert!(std::panic::catch_unwind(|| {
         eprintln!("{}", A);
     })

src/test/ui/threads-sendsync/task-stderr.rs

@@ -1,7 +1,7 @@
 // run-pass
 // ignore-emscripten no threads support
 
-#![feature(set_stdio)]
+#![feature(internal_output_capture)]
 
 use std::io;
 use std::str;
@@ -13,7 +13,7 @@ fn main() {
     let res = thread::Builder::new().spawn({
         let data = data.clone();
         move || {
-            io::set_panic(Some(data));
+            io::set_output_capture(Some(data));
             panic!("Hello, world!")
         }
     }).unwrap().join();