samples: posix: eventfd: Use NET_TEST config

NET_TEST is the approprite config for networking tests that do not
require a network device. This has the same effect of disabling
NET_SLIP_TAP, but also disables other network devices. This is required
to disable e1000 Ethernet driver on the qemu_cortex_a53 board.

Signed-off-by: Grant Ramsay <gramsay@enphaseenergy.com>