Adds AshTest

ash/mus/test/wm_test_base.h

Index: ash/mus/test/wm_test_base.h
diff --git a/ash/mus/test/wm_test_base.h b/ash/mus/test/wm_test_base.h
index 913bf5fd68b3cecd5d9321b9527337f0163bd3f1..8c17d39f29eff12eae1f9b1ad7ff7056c9f8e89e 100644
--- a/ash/mus/test/wm_test_base.h
+++ b/ash/mus/test/wm_test_base.h
@@ -27,6 +27,7 @@ class Window;
 namespace ash {
 namespace mus {
 
+class AshTestImplMus;
 class RootWindowController;
 class WmTestHelper;
 
@@ -69,6 +70,8 @@ class WmTestBase : public testing::Test {
   void TearDown() override;
 
  private:
+  friend class AshTestImplMus;
+
   bool setup_called_ = false;
   bool teardown_called_ = false;
   std::unique_ptr<WmTestHelper> test_helper_;