Add: Mock sprite cache intialization.

This fills up the sprite cache with SPR_OPENTTD_BASE + OPENTTD_SPRITE_COUNT zero-size sprites, to
allow GetSpriteSize() calls to continue from unit-tests.

src/tests/mock_spritecache.h

@@ -0,0 +1,15 @@
+/*
+ * This file is part of OpenTTD.
+ * OpenTTD is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, version 2.
+ * OpenTTD is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+ * See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with OpenTTD. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+/** @file mock_spritecache.h Mock sprite cache definition. */
+
+#ifndef MOCK_SPRITECACHE_H
+#define MOCK_SPRITECACHE_H
+
+void MockGfxLoadSprites();
+
+#endif /* MOCK_SPRITECACHE_H */