net: tcp2: Randomize the initial sequence number

The initial sequence number for a connection should be randomized
to prevent easy guesses.

Do not randomize the sequence number if network test or
test protocol is enabled.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Signed-off-by: Oleg Zhurakivskyy <oleg.zhurakivskyy@intel.com>