Add unit tests for socket dispatcher module. The tests verify that:
* a socket is automatically dispatched to a regular socket on any
first socket call, if the socket was not bound already before
(according to the default priority rules)
* a socket is correctly dispatched to an offloaded/native socket
implementation after a SO_BINDTODEVICE call
* a socket is correctly dispatched to a native TLS socket, with an
underlying native/offloaded socket, if TLS_NATIVE and SO_BINDOTODEVICE
socket options are used
Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>
6da10468ad