Attempt to fix build of unit tests on GCC 4.8

The tests were building just fine on Clang and GCC 7, but not on GCC 4.8
that Travis CI uses. That points towards a compiler bug. As a workaround,
simply initialize the widgets in a more straightforward way.

src/tests/gui/iterator.cpp

@@ -121,11 +121,9 @@ static void add_widget(gui2::grid& grid
 			, 0);
 }
 
-template<class T, typename... T2>
-static void test_control(T2&&... args)
+template<class T>
+static void test_control(T&& control)
 {
-	T control(std::forward<T2>(args)...);
-
 	{
 		gui2::iteration::iterator< gui2::iteration::policy::order::top_down<
 				true
@@ -171,14 +169,14 @@ static void test_control(T2&&... args)
 static void test_control()
 {
 	/* Could add more widgets to the list. */
-	test_control<gui2::label>(gui2::implementation::builder_label(config()));
+	test_control(gui2::label(gui2::implementation::builder_label(config())));
 
 }
 
 static void test_grid()
 {
 	/* An empty grid behaves the same as a control so test here. */
-	test_control<gui2::grid>();
+	test_control(gui2::grid());
 
 	/* Test the child part here. */
 	gui2::grid grid(2 ,2);