GN: Build rules and fixes for (almost) all mojo unittest targets

mojo/public/cpp/utility/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.
+
+# GYP version: mojo/mojo_public_tests.gypi:mojo_public_utility_unittests
+test("mojo_public_utility_unittests") {
+  deps = [
+    "//mojo/common/test:run_all_unittests",
+    "//mojo/public/cpp/test_support:test_utils",
+    "//mojo/public/cpp/utility",
+    "//testing/gtest",
+  ]
+
+  sources = [
+    "mutex_unittest.cc",
+    "run_loop_unittest.cc",
+    "thread_unittest.cc",
+  ]
+}