tests: cleanup tests/ui/process/try-wait.rs

- Remove already stable feature gate and remove
  `#![allow(stable_features)]`.
- Replace `ignore-*` with `needs-subprocess`.

tests/ui/process/try-wait.rs

@@ -1,9 +1,5 @@
 //@ run-pass
-
-#![allow(stable_features)]
-//@ ignore-wasm32 no processes
-//@ ignore-sgx no processes
-#![feature(process_try_wait)]
+//@ needs-subprocess
 
 use std::env;
 use std::process::Command;