Add first test cases for netsnmpagent SNMP objects (Unsigned32)

This first set of test cases tests
- SNMPGET on a read/write Unsigned32 object without initval returns 0
- SNMPSET of that object to 42 raises no exception
- SNMPGET on that object then returns 42
- SNMPGET on a Unsigned32 object with initval 0 returns 0
- SNMPGET on a Unsigned32 object with initval -1 returns 4294967295
- SNMPGET on a Unsigned32 object with initval 4294967295 returns 4294967295
- SNMPGET on a read-only Unsigned32 object without initval returns 0
- SNMPSET on that object raises a NotWritableError exception
Do not call net-snmp's shutdown_agent() anymore

Unfortunately, the situation is even worse than described in 9c6c5560 so that
we'll have to revert that change. Calling shutdown_agent() will cause trouble
if SNMP objects have been registered (double free()s).