test/interrupt: account for race with callback

Because the eal interrupt framework can race when invoking the callback
and a separate unregister call, the test needs to accommodate the chance
that the two collide.  Do this by checking the return value of unregister
against the race-condition flag (EAGAIN).

Fixes: f1a6c22424ce ("app/test: update interrupts test")
Cc: stable@dpdk.org
Signed-off-by: Aaron Conole <aconole@redhat.com>
Reviewed-by: David Marchand <david.marchand@redhat.com>