tests: cleanup tests/ui/command/command-exec.rs

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

tests/ui/command/command-exec.rs

@@ -1,13 +1,9 @@
 //@ run-pass
 
-#![allow(stable_features)]
-//@ ignore-windows - this is a unix-specific test
-//@ ignore-wasm32 no processes
-//@ ignore-sgx no processes
+//@ only-unix (this is a unix-specific test)
+//@ needs-subprocess
 //@ ignore-fuchsia no execvp syscall provided
 
-#![feature(process_exec)]
-
 use std::env;
 use std::os::unix::process::CommandExt;
 use std::process::Command;