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/CMakeLists.txt

@@ -2,6 +2,8 @@ add_test_files(
     landscape_partial_pixel_z.cpp
     math_func.cpp
     mock_fontcache.h
+    mock_spritecache.cpp
+    mock_spritecache.h
     string_func.cpp
     strings_func.cpp
     test_main.cpp