Disable XTest by default

On Ubuntu 24.04 running XWayland, XTestFakeMotionEvent() appears to succeed but doesn't actually move the cursor, even if the cursor is hidden.

src/video/x11/SDL_x11xtest.c

@@ -30,7 +30,8 @@ static bool xtest_initialized = false;
 
 void X11_InitXTest(SDL_VideoDevice *_this)
 {
-#ifdef SDL_VIDEO_DRIVER_X11_XTEST
+// This is currently disabled since it doesn't appear to work on XWayland
+#if 0//def SDL_VIDEO_DRIVER_X11_XTEST
     Display *display = _this->internal->display;
     int event, error;
     int opcode;