SNMP assumes the first half of the queues to be unicast and the second half to be multicast.
This assumption was fine for most hwSkus but not new ones being added to
support, i.e. x86_64-arista_7060x6

Only tested on Arista hwSkus.

Added a unit test subcase and relevant mock tablesto check for SNMP
behavior when there are more unicast queues than half of the total
number of queues.

This is to ensure SNMP does not operate on the false assumption where
there is always a 50-50 split between unicast and multicast queues.