unit: Add register with invalid payload length
unit: Add register with invalid payload length

This patch adds a unit test to verify if the gateway handles properly
a malformed registration message. This unit test verifies if the gateway
catches payload with invalid length.

CESARBR/knot-service-source,CESARBR/knot-service-source,CESARBR/knot-service-source,CESARBR/knot-service-source,CESARBR/knot-service-source