Re: sys_socket tests: Improve tests for macro definitions.

[Bruno Haible]

Collin Funk wrote:
> Adding a few tests based on longstanding POSIX specification (i.e. not
> added in 2024).
> 
> * AF_UNSPEC should be 0.
> * All supported AF_* constants must have distinct values.
> * SHUT_RD, SHUT_RDWR, SHUT_WR must have distinct values.

Thanks!