Add signal test for each supported signal.

tests/standalone/io/signal_test_script.dart

+// Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import "dart:io";
+
+void main(args) {
+  var signal;
+  switch (args[0]) {
+    case 'SIGHUP': signal = ProcessSignal.SIGHUP; break;
+    case 'SIGINT': signal = ProcessSignal.SIGINT; break;
+    case 'SIGTERM': signal = ProcessSignal.SIGTERM; break;
+    case 'SIGUSR1': signal = ProcessSignal.SIGUSR1; break;
+    case 'SIGUSR2': signal = ProcessSignal.SIGUSR2; break;
+    case 'SIGWINCH': signal = ProcessSignal.SIGWINCH; break;
+  }
+  signal.watch().first.then((s) {
+    if (signal != s) exit(1);
+    if (signal.toString() != args[0]) exit(1);
+    print('got signal');
+  });
+  print("ready");
+}
+