IRQ latency can be ridiculously 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>