boards: mps2_an385: Exclude platform from networking tests

The platform is known for having unstable system timer with networking
enabled (see https://github.com/zephyrproject-rtos/zephyr/issues/48608

)
causing occasional failures of time-sensitive networking testsuties
(TLS, now DHCPv6). Instead of excluding the platform on per-test basis,
just exclude the platform from networking testing globally.

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