tests: net: socketpair: use do not alias read and write

* do not define `read`, `write`, as different symbols
* use `recv`, and `send` as defined by
  `CONFIG_NET_SOCKETS_POSIX_NAMES`
* do not specify `(2)` as manual-page sections are
  somewhat irrelevant

Note: we use `CONFIG_NET_SOCKETS_POSIX_NAMES` here, and
`CONFIG_POSIX_API` in `samples/` to show that the socketpair
API is available in both configurations.

Signed-off-by: Chris Friedt <cfriedt@meta.com>