Makes AshTestImpl::Create create the right impl based on config

ash/test/ash_test_impl_aura.cc

Index: ash/test/ash_test_impl_aura.cc
diff --git a/ash/test/ash_test_impl_aura.cc b/ash/test/ash_test_impl_aura.cc
index ec4fa7843c5bbbf5024bfa9ecf5940431228185e..75fb8ec3d530db4af4e4e5aa60279e378a0865b0 100644
--- a/ash/test/ash_test_impl_aura.cc
+++ b/ash/test/ash_test_impl_aura.cc
@@ -6,9 +6,12 @@
 
 #include "ash/common/test/ash_test.h"
 #include "ash/common/wm_window.h"
+#include "ash/mus/test/ash_test_impl_mus.h"
+#include "ash/public/cpp/config.h"
 #include "ash/screen_util.h"
 #include "ash/shell.h"
 #include "ash/test/ash_test_base.h"
+#include "ash/test/ash_test_helper.h"
 #include "base/memory/ptr_util.h"
 #include "ui/aura/test/test_window_delegate.h"
 #include "ui/display/display_layout.h"
@@ -101,7 +104,10 @@ void AshTestImplAura::AddTransientChild(WmWindow* parent, WmWindow* window) {
 
 // static
 std::unique_ptr<AshTestImpl> AshTestImpl::Create() {
-  return base::MakeUnique<AshTestImplAura>();
+  if (test::AshTestHelper::config() == Config::CLASSIC)
+    return base::MakeUnique<AshTestImplAura>();
+
+  return base::MakeUnique<mus::AshTestImplMus>();
 }
 
 }  // namespace ash