tests: net: sockets: misc: Fix SO_BINDTODEVICE test

The test case for SO_BINDTODEVICE socket option was flaky, the client
socket always sent the datagram to the IP address of the second
interface, so in theory every packet should end up on that interface.
In practice though, due to imperfect loopback packet handling, the test
worked as the packet ended up on the interface it was sent from.

The test should send datagrams to the IP addresses of the interface 1
and 2 alternatively. The server socket binds to ANY address, so w/o
interface binding it should receive all datagrams, so it allows to
verify if SO_BINDTODEVICE filtering works fine.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>