It does this:
  // Verify that the logout confirmation dialog is not showing.
  ash::LogoutConfirmationController* logout_confirmation_controller =
      ash::Shell::Get()->logout_confirmation_controller();
  ASSERT_TRUE(logout_confirmation_controller);
  EXPECT_FALSE(logout_confirmation_controller->dialog_for_testing());

I think we need a mojo test API for this. All the test of the tests in this suite pass.