drivers: ethernet: Disable drivers if tests are enabled

The network tests at tests/net use simulated network interfaces
and set CONFIG_NET_TEST to indicate that. If the config option
is set, then we do not want any extra Ethernet driver to
complicate the testing scenario so all external Ethernet
network interfaces should be disabled.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>