tests: fix unix domain socket decoding availability test

Since inet_diag and unix_diag interfaces are implemented by different
kernel modules, they have to be tested separately.

* tests/netlink_unix_diag.c: New file.
* tests/unix-yy.test: Use it.
* tests/Makefile.am (check_PROGRAMS): Add it.
* tests/.gitignore: Likewise.
diff --git a/tests/unix-yy.test b/tests/unix-yy.test
index 6e7bd56..1114e17 100755
--- a/tests/unix-yy.test
+++ b/tests/unix-yy.test
@@ -17,11 +17,11 @@
 ./net-accept-connect ||
 	fail_ 'net-accept-connect failed'
 
-./netlink_inet_diag || {
+./netlink_unix_diag || {
 	if [ $? -eq 77 ]; then
-		framework_skip_ 'NETLINK_INET_DIAG is not available'
+		framework_skip_ 'NETLINK_SOCK_DIAG for unix domain sockets is not available'
 	else
-		fail_ 'netlink_inet_diag failed'
+		fail_ 'netlink_unix_diag failed'
 	fi
 }