Prepare a set of unit tests for snmp functionality

- SnmpCommand: get, set, getbulk
  - variable adding
  - attribute setting
  - correct snmp packet size generating
- SnmpTask: single request/response
  - unique id generation
  - attribute setting
  - correct callback execution
  - correct task state and attributes in a callback
  - cancelling
- SnmpManager
  - basic single request/response
  - executing command
  - executing user-defined task
  - changing factory attributes, then executing commands
  - several concurrent requests
  - stopping application (should cancel all tasks)
  - chaining several requests through callback

Attribute setting testing means not only to check that the attribute can be set, but to check that it actually changes object's behavior.