tests/samples: net: Fix the max number of file descriptors

Make sure that CONFIG_ZVFS_OPEN_MAX in the tests and samples is at
least the same value as CONFIG_NET_SOCKETS_POLL_MAX so that we
cannot have more polled sockets than there are available file
descriptors.

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