Add a specific test for spawn() returning ENOENT

src/test/run-pass/process-spawn-nonexistent.rs

@@ -0,0 +1,23 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// ignore-cloudabi no processes
+// ignore-emscripten no processes
+
+use std::io::ErrorKind;
+use std::process::Command;
+
+fn main() {
+    assert_eq!(Command::new("nonexistent")
+                   .spawn()
+                   .unwrap_err()
+                   .kind(),
+               ErrorKind::NotFound);
+}