test: Rename interfaces defined in checksum offload test

The struct net_if object placement in linker 'net_if_area' section
now depends on the order of ETH_NET_DEVICE_INIT() macros in the source
code (for the same object file - like app/libapp.a(main.c.obj)).

Currently the 'eth_offloading_enabled_test' device is placed first,
followed by 'eth_offloading_disabled_test'.

This placement is important when one would like to access those
interfaces with net_if_get_default() function.

After the rename the order is explicit, when we append the device name
to the ._net_if.static.<dev name> object in linker's net_if_area.

The code now works as all struct net_if interfaces are placed as
'._net_if.static.net_if' objects.

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