Move remaining test targets out of //mojo/public.

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.
 
-# GYP version: mojo/mojo_base.gyp:mojo_public_system_unittests
-test("mojo_public_system_unittests") {
-  deps = [
-    "//mojo/edk/test:run_all_unittests",
-    "//mojo/public/c/system/tests",
-    "//mojo/public/cpp/system",
-    "//mojo/public/cpp/test_support:test_utils",
-    "//testing/gtest",
-  ]
+import("../../../mojo_sdk.gni")
+
+mojo_sdk_source_set("tests") {
+  testonly = true
 
   sources = [
     "core_unittest.cc",
     "macros_unittest.cc",
   ]
+
+  deps = [
+    "//testing/gtest",
+  ]
+
+  mojo_sdk_deps = [
+    "mojo/public/c/system/tests",
+    "mojo/public/cpp/system",
+    "mojo/public/cpp/test_support:test_utils",
+  ]
 }