Include functional in actor tests to fix iwyu errors

tests/AlarmActorTest.cpp

 // SPDX-License-Identifier: LGPL-3.0-or-later
 // Copyright © 2020 Florian Fischer
-#include <algorithm>	// for copy
-#include <cstdlib>		// for exit, EXIT_FAILURE, EXIT_SUC...
-#include <iostream>		// for operator<<, basic_ostream
+#include <algorithm>	 // for copy
+#include <cstdlib>		 // for exit, EXIT_FAILURE, EXIT_SUC...
+#include <functional>	 // bind
+#include <iostream>		 // for operator<<, basic_ostream
 
 #include "Actor.hpp"										 // for Actor
 #include "BinaryPrivateSemaphore.hpp"		 // for BPS

tests/SimpleActorTest.cpp

 // SPDX-License-Identifier: LGPL-3.0-or-later
 // Copyright © 2020 Florian Schmaus
-#include <algorithm>	// for copy
-#include <atomic>			// for atomic_thread_fence, memory_...
-#include <cstdint>		// for uint64_t
-#include <cstdlib>		// for exit, EXIT_FAILURE, EXIT_SUC...
-#include <iostream>		// for operator<<, basic_ostream
+#include <algorithm>	 // for copy
+#include <atomic>			 // for atomic_thread_fence, memory_...
+#include <cstdint>		 // for uint64_t
+#include <cstdlib>		 // for exit, EXIT_FAILURE, EXIT_SUC...
+#include <functional>	 // for bind
+#include <iostream>		 // for operator<<, basic_ostream
 
 #include "Actor.hpp"										 // for Actor
 #include "CountingPrivateSemaphore.hpp"	 // for CPS

tests/TellActorFromAnywhereTest.cpp

 // SPDX-License-Identifier: LGPL-3.0-or-later
 // Copyright © 2020 Florian Schmaus
-#include <algorithm>	// for copy, copy_backward
-#include <cstdlib>		// for exit, EXIT_FAILURE, EXIT_SUCCESS
+#include <algorithm>	 // for copy, copy_backward
+#include <cstdlib>		 // for exit, EXIT_FAILURE, EXIT_SUCCESS
+#include <functional>	 // for bind
 #include <thread>
 
 #include "Actor.hpp"