Use ARC for Suggestions tests

ios/chrome/browser/ui/suggestions/BUILD.gn

 # Copyright 2016 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("suggestions") {
   configs += [ "//build/config/compiler:enable_arc" ]
   sources = [
     "suggestions_article_item.h",
     "suggestions_article_item.mm",
     "suggestions_collection_updater.h",
@@ skipping 14 matching lines @@
     "//ios/chrome/browser/ui/collection_view",
     "//ios/third_party/material_roboto_font_loader_ios",
     "//ui/base",
   ]
   public_deps = [
     "//ios/third_party/material_components_ios",
   ]
 }
 
 source_set("unit_tests") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   testonly = true
   sources = [
     "suggestions_article_item_unittest.mm",
     "suggestions_expandable_item_unittest.mm",
     "suggestions_item_unittest.mm",
   ]
   deps = [
     ":suggestions",
     "//ios/chrome/browser/ui/collection_view",
     "//testing/gtest",
     "//third_party/ocmock",
   ]
 }