test: net: ipv6: Cleanup Ethernet test interface operation

One of the test interfaces is defined as Ethernet interface, however it
was missing ethernet_init() call, hence for example interface flags were
uninitialized. This can cause issues with multicast tests, as
NET_L2_MULTICAST flag was missing on the interface.

Additionally, Ethernet-type interfaces should use struct
ethernet_context as a base for their context data, otherwise Ethernet L2
will reach to some rubbish memory locations.

Finally, since the interface now defines correct flags, all-nodes
multicast address is now added to the interface automatically as a part
of ND logic. Therefore, skip adding that address manually from
ipv6_setup().

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