selftests: af_unix: Add tests for ECONNRESET and EOF semantics

selftests: af_unix: Add tests for ECONNRESET and EOF semantics

Add selftests to verify and document Linux’s intended behaviour for
UNIX domain sockets (SOCK_STREAM and SOCK_DGRAM) when a peer closes.
The tests verify that:

 1. SOCK_STREAM returns EOF when the peer closes normally.
 2. SOCK_STREAM returns ECONNRESET if the peer closes with unread data.
 3. SOCK_SEQPACKET returns EOF when the peer closes normally.
 4. SOCK_SEQPACKET returns ECONNRESET if the peer closes with unread data.
 5. SOCK_DGRAM does not return ECONNRESET when the peer closes.

This follows up on review feedback suggesting a selftest to clarify
Linux’s semantics.

Suggested-by: Kuniyuki Iwashima <kuniyu@google.com>
Signed-off-by: Sunday Adelodun <adelodunolaoluwa@yahoo.com>
Link: https://patch.msgid.link/20251113112802.44657-1-adelodunolaoluwa@yahoo.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>