Test: Avoid leaking widget tree in WindowDescTestsFixture

src/tests/test_window_desc.cpp

@@ -98,4 +98,5 @@ TEST_CASE_METHOD(WindowDescTestsFixture, "WindowDesc - NWidgetPart validity")
 
 	REQUIRE_NOTHROW(root = MakeWindowNWidgetTree(window_desc->nwid_begin, window_desc->nwid_end, &biggest_index, &shade_select));
 	CHECK((root != nullptr));
+	delete root;
 }