3rdParty/SDL2: Disable SDL_TEST

The fully-vendored source distribution removes all test files to reduce
the size, which causes a CMake configuration error if `SDL_TEST` is on.

3rdParty/SDL2/CMakeLists.txt

@@ -10,6 +10,7 @@ else()
   set(SDL_SHARED ON)
   set(SDL_STATIC OFF)
 endif()
+set(SDL_TEST OFF)
 
 include(functions/FetchContent_MakeAvailableExcludeFromAll)
 include(FetchContent)