tests: cleanup tests/ui/process/sigpipe-should-be-ignored.rs

- Unwrap a must-use I/O result and remove `#![allow(unused_must_use)]`.
- Replace `ignore-*` with `needs-subprocess`.

tests/ui/process/sigpipe-should-be-ignored.rs

@@ -1,12 +1,9 @@
 //@ run-pass
+//@ needs-subprocess
 
-#![allow(unused_must_use)]
 // Be sure that when a SIGPIPE would have been received that the entire process
 // doesn't die in a ball of fire, but rather it's gracefully handled.
 
-//@ ignore-wasm32 no processes
-//@ ignore-sgx no processes
-
 use std::env;
 use std::io::prelude::*;
 use std::io;
@@ -14,7 +11,7 @@
 
 fn test() {
     let _ = io::stdin().read_line(&mut String::new());
-    io::stdout().write(&[1]);
+    io::stdout().write(&[1]).unwrap();
     assert!(io::stdout().flush().is_err());
 }