tests: net: socketpair: skip nonblocking tests on posix arch

The `fcntl()` call (`zsock_fcntl()`) does not seem to work
properly (specifically with `CONFIG_ARCH_POSIX`). This will
be resolved in subsequent work harmonizing the networking
subsystem and POSIX library.

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