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

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

Index: mojo/public/cpp/system/tests/BUILD.gn
diff --git a/mojo/public/cpp/system/tests/BUILD.gn b/mojo/public/cpp/system/tests/BUILD.gn
new file mode 100644
index 0000000000000000000000000000000000000000..0c629a1b9780d12584cef15e893ed8a71d00f731
--- /dev/null
+++ b/mojo/public/cpp/system/tests/BUILD.gn
@@ -0,0 +1,18 @@
+# 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_system_unittests
+test("mojo_public_system_unittests") {
+  deps = [
+    "//mojo/common/test:run_all_unittests",
+    "//mojo/public/c/system/tests",
+    "//mojo/public/cpp/test_support:test_utils",
+    "//testing/gtest",
+  ]
+
+  sources = [
+    "core_unittest.cc",
+    "macros_unittest.cc",
+  ]
+}