better support for posix api read write in socketpair tests

[cfriedt]

Is your enhancement proposal related to a problem? Please describe.
Use READ / WRITE macros in socketpair tests, to abstract read / recv and write / send as in #25356

socketpair should work with unistd read / write when

#if !defined(__ZEPHYR__) || defined(CONFIG_POSIX_API)

is true

Describe the solution you'd like
I'll throw together a patch after #25271

Describe alternatives you've considered

Additional context

[cfriedt]

IIRC, this should Just Work already - will need to double check.