[U-Boot] Driver model test breakages

Hi,

The following three commits causes breakages in the driver model tests:

4efad20a sf: Update status reg check in spi_flash_cmd_wait_ready 45b47734 net/arp: account for ARP delay, avoid duplicate packets on timeout 9961a0b6 sandbox: add a sandbox timer and basic test

Can you please take a look? You can run them with ./test/dm/test-dm.sh

The last one looks a bit complicated - I suspect it will be harder to fix. I am going to try to come up with some ideas...

Regards, Simon