No that we load MIB files at snmpd start-up chances that the sleep build

No that we load MIB files at snmpd start-up chances that the sleep build
into the regress test don't suffice anymore for slow regress machines.
(Ab)use the agentx socket (which gets created after all the MIB files
have been parsed) to detect if snmpd is available. For now we poll at
0.1s intervals for a total of 100 tries.

found by and earlier diff tested by and OK anton@