Mojo: Move waiting APIs to public library

mojo/public/cpp/system/tests/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 source_set("tests") {
   testonly = true
 
   sources = [
     "core_unittest.cc",
     "handle_signals_state_unittest.cc",
     "simple_watcher_unittest.cc",
+    "wait_set_unittest.cc",
+    "wait_unittest.cc",
   ]
 
   deps = [
     "//base",
     "//mojo/public/c/system/tests",
     "//mojo/public/cpp/system",
     "//mojo/public/cpp/test_support:test_utils",
     "//testing/gtest",
   ]
 }