tests: net: sockets: Update packet socket tests

Fix the existing DGRAM packet socket test. The test will now specify
LL destination address and protocol type. Also, update the fake ethernet
device send handler, so that it sets the packet network interface
corretly, so that the packet is not rejected at Ethernet L2.

Additionally, add two additional test cases verifying datagram packet
sockets. Fist test will exchange packet between RAW and DGRAM socket,
making sure that net stack forwards/removes L2 header correctly. The
second test verifies that it's possible to recieve a packet on both RAW
and DGRAM socket, making sure the received data has correct format.

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