Add test case to verify SNMP counters

[vitthalmagadum]

Trap PDUs counter should be a non-zero value

sample output:

leaf1-dc1#show snmp | json
{
"counters": {
"serviceEnabled": true,
"inPkts": 0,
"inVersionErrs": 0,
"inBadCommunityNames": 0,
"inBadCommunityUses": 0,
"inParseErrs": 0,
"inRequestVars": 0,
"inSetVars": 0,
"inGetPdus": 0,
"inGetNextPdus": 0,
"inSetPdus": 0,
"outPkts": 26,
"outTooBigErrs": 0,
"outNoSuchNameErrs": 0,
"outBadValueErrs": 0,
"outGeneralErrs": 0,
"outGetResponsePdus": 0,
"outTrapPdus": 26,
"outTrapDrops": 0,
"groupLength": 0,
"numberOfGroups": 0,
"numberOfViews": 0
},