Switch to std::unique_ptr<DummyExecutor>(new DummyExecutor())

Windows marks std::make_unique as [[nodiscard]], which causes
a build warning there.  Fake it out by using std::unique_ptr
with a new instead.

Signed-off-by: Chris Lalancette <clalancette@openrobotics.org>

rclcpp/test/rclcpp/test_executor.cpp

@@ -122,14 +122,14 @@ TEST_F(TestExecutor, constructor_bad_guard_condition_init) {
   auto mock = mocking_utils::patch_and_return(
     "lib:rclcpp", rcl_guard_condition_init, RCL_RET_ERROR);
   EXPECT_THROW(
-    std::make_unique<DummyExecutor>(),
+    std::unique_ptr<DummyExecutor>(new DummyExecutor()),
     rclcpp::exceptions::RCLError);
 }
 
 TEST_F(TestExecutor, constructor_bad_wait_set_init) {
   auto mock = mocking_utils::patch_and_return("lib:rclcpp", rcl_wait_set_init, RCL_RET_ERROR);
   RCLCPP_EXPECT_THROW_EQ(
-    std::make_unique<DummyExecutor>(),
+    std::unique_ptr<DummyExecutor>(new DummyExecutor()),
     std::runtime_error("Failed to create wait set in Executor constructor: error not set"));
 }