net: mqtt-sn: Update MQTT-SN Unit Tests

Fixes: #78010
This commit adds tests for the "Gateway Advertisement and Discovery"
process. This includes tests for handling SEARCHGW, GWINFO, and ADVERTISE
as well as manually adding a Gateway for use. Gateway pruning when
missing ADVERTISE messages is also tested.

Signed-off-by: Kenneth Witham <kennywitham4@gmail.com>