tests: robustify unix-yy.test

tests: robustify unix-yy.test

Implement additional synchronization between parent and child processes
to guarantee that the child starts closing connected socket only after
exiting of the parent's accept() syscall.
This guarantee seems to be necessary to reliably receive UNIX_DIAG_PEER
messages from NETLINK_SOCK_DIAG interface.

* tests/net-accept-connect.c: Implement additional synchronization
between parent and child processes.
* tests/unix-yy-connect.awk: Update.