high for various reasons, so our
current timeouts of 100 ms or 10 ms are too short.

Change the IRQ and event tests to use polling loops starting with a
delay of 1 tick and doubling that if necessary up to a maximum total
delay of approximately 1 second.

Raise the loopback packet RX timeout to 1 second.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>