New classes covered with unit tests: SocketAddress, EthernetInterface, EMACInterface, WiFiAccessPoint.
Also added missing namespace in front of mbed::Callback usages.
EMACInterface is covered by the EthernetInterface unit tests.
Improved the stubs for event queue and nsapi_dns, to allow checking if callback are handled correctly. This involves some memory allocation and deallocation.
The NetworkStackWrapper is not covered as it seems to be deprecated code.