tests: add compatibility layer for accept call

tests: add compatibility layer for accept call

Recent glibc (since version 2.26) uses accept4 syscall for implementing
accept call on sparc.  Unfortunately, it's impossible to simply fall
back on raw syscall as it had not been wired up until linux commit
v4.4-rc8~4^2~1.

* tests/accept_compat.h: New file.
* tests/Makefile.am (EXTRA_DIST): Add it.
* tests/net-y-unix.c: Include accept_compat.h, use do_accept()
instead of accept() calls.
* tests/net-yy-inet.c: Likewise.
* tests/net-yy-unix.c: Likewise.
* tests/net.expected: Allow accept4.