unittests: add test network device

The reasoning behind this commit is twofold:
1. test the "lower" part (the device part) of netdev
2. provide a well defined testing infrastructure for modules that utilise
   netdev
The actual tests that represent 1. are provided in the following commit
unittests: Provide unittests for net_dev_dummy

* tests if net_dev_dummy is correct and if driver part of net_dev is
  correct
Merge pull request #1731 from authmillenon/netdev-test-dummy

netdev: Provide test network device with netdev interface