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.
Added more tests, improved the existing ones. setblocking tests were not checking anything, so they were removed and these functions are called in TCPSocket tests instead.
Add functional and line coverage for UDPSocket and TCPSocket. The EventFlagsstub and NetworkStackstub classes are allowed to store multiple return values to allow running internal loops multiple times.