tests: cleanup tests/ui/panic-runtime/abort.rs

- Ignore unused value, remove `#![allow(unused_variable)]`.
- Replace `ignore-*` with `needs-subprocess`.

tests/ui/panic-runtime/abort.rs

@@ -1,9 +1,7 @@
 //@ run-pass
-#![allow(unused_variables)]
 //@ compile-flags:-C panic=abort
 //@ no-prefer-dynamic
-//@ ignore-wasm32 no processes
-//@ ignore-sgx no processes
+//@ needs-subprocess
 
 use std::env;
 use std::process::Command;
@@ -18,7 +16,7 @@ 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" {