tests: cleanup tests/ui/panic-runtime/lto-unwind.rs

- Ignore an unused variable and remove `#![allow(unused_variables)]`.
- Replace `ignore-*` with `needs-subprocess`.

Author: jieyouxu

tests/ui/panic-runtime/lto-unwind.rs

@@ -1,11 +1,8 @@
 //@ run-pass
-#![allow(unused_variables)]
-
 //@ compile-flags:-C lto -C panic=unwind
 //@ needs-unwind
 //@ no-prefer-dynamic
-//@ ignore-emscripten no processes
-//@ ignore-sgx no processes
+//@ needs-subprocess
 
 use std::process::Command;
 use std::env;
@@ -20,7 +17,7 @@ impl Drop for Bomb {
 
 fn main() {
     let mut args = env::args_os();
-    let me = args.next().unwrap();
+    let _ = args.next().unwrap();
 
     if let Some(s) = args.next() {
         if &*s == "foo" {