AM3358: WDT in U-boot

Assuming by testing you mean seeing the watchdog timeout to confirm it is doing its job? For this you could just need to insert an endless loop at a suitable place in U-Boot, such as into the run_main_loop() function in common/board_r.c:

static int run_main_loop(void)
{
#ifdef CONFIG_SANDBOX
	sandbox_main_loop_init();
#endif
	/* main_loop() can return to retry autoboot, if so just run it again */
	for (;;)
		main_loop();
	return 0;
}

In the above, if you comment out the invocation for main_loop(), you won't be periodically petting the watchdog, and the device should reset after a while.

Regards, Andreas