check_register: disable OPTIONS keepalives for the validation timeout test
check_register: disable OPTIONS keepalives for the validation timeout test

Only validation requests seem to be regarded in testing, so retransmission
of plain keepalive probes interferes with the check timing when keepalive
timer is made deferrable.

BelledonneCommunications/sofia-sip,unispeech/sofia-sip,erdincay/sofia-sip,unispeech/sofia-sip,xhook/sofia-sip,erdincay/sofia-sip,erdincay/sofia-sip,BelledonneCommunications/sofia-sip,unispeech/sofia-sip,xhook/sofia-sip,erdincay/sofia-sip,xhook/sofia-sip,BelledonneCommunications/sofia-sip,unispeech/sofia-sip,xhook/sofia-sip,xhook/sofia-sip,BelledonneCommunications/sofia-sip