tests: cleanup tests/ui/process/signal-exit-status.rs

- Replace `ignore-windows` -> `only-unix` since the test exercises Unix
  signals and `ExitStatus::code` behavior that's specific to Unix.
- Replace `ignore-*` with `needs-subprocess`.

tests/ui/process/signal-exit-status.rs

@@ -1,7 +1,6 @@
 //@ run-pass
-//@ ignore-wasm32 no processes
-//@ ignore-sgx no processes
-//@ ignore-windows
+//@ needs-subprocess
+//@ only-unix (`code()` returns `None` if terminated by a signal on Unix)
 //@ ignore-fuchsia code returned as ZX_TASK_RETCODE_EXCEPTION_KILL, FIXME (#58590)
 
 #![feature(core_intrinsics)]