chore: add missing SPDX tags

Follow-up to 99ceb03a1c

src/tests/core/core_timing.cpp

@@ -1,6 +1,5 @@
-// Copyright 2016 Dolphin Emulator Project / 2017 Dolphin Emulator Project
-// Licensed under GPLv2+
-// Refer to the license.txt file included.
+// SPDX-FileCopyrightText: 2016 Dolphin Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
 
 #include <catch2/catch.hpp>