Changes

PCU_Tests_SNS: SNS Add: handle NS_Alive while waiting for SNS_Ack

Similiar to 61ccea9ecadc ("PCU_Tests_SNS: del/change weight: don't fail on NS"),
the SNS Add procedure might have to handle a NS Alive PDU in the receiving queue while waiting
for a SNS ACK.

Related: OS#6611
Change-Id: I10646d9acfe280ddfb8852a4d5c37882aae99874