samples: net: sockets: Allow to build and test with POSIX subsys

With CONFIG_POSIX_API enabled, these samples now build under Zephyr
with exactly the same source as e.g. Linux (or in general, other POSIX
systems). However, building without CONFIG_POSIX_API (i.e. with
CONFIG_NET_SOCKETS_POSIX_NAMES aux option) is retained for now.

Add testcase definitions to build these samples with CONFIG_POSIX_API
in CI.

Fixes: #17353

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>