test: Lookup for struct net_if interface defined in ipv6 test

The tests/net/ipv6/src/main.c uses net_if_get_default() to get the
interface for testing. This is problematic when board introduces its own
linker script, which places board's network interface on the "default"
position - i.e. the first one.

To fix this issue - the lookup for network device defined for this test
is used instead.

Moreover, interfaces rename has been performed in a manner, which would
allow working with 'eth_native_posix' interface being added as linker
object when drivers/ethernet/eth_native_posix.c file is linked for
./zephyr/tests/net/ipv6 test on 'native_posix' board.

Signed-off-by: Lukasz Majewski <lukma@denx.de>