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

- Remove already stabilized feature gate and
  `#![allow(stable_features)]`.
- Convert `ignore-windows` to `only-unix`.
- Convert `ignore-*` to `needs-subprocess`.

tests/ui/command/command-pre-exec.rs

@@ -1,11 +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
-#![feature(process_exec, rustc_private)]
+
+#![feature(rustc_private)]
 
 extern crate libc;