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/.gitignore b/tests/.gitignore
index 9504dd3..b53ef6a 100644
--- a/tests/.gitignore
+++ b/tests/.gitignore
@@ -3,6 +3,7 @@
 mmsg
 net-accept-connect
 netlink_inet_diag
+netlink_unix_diag
 scm_rights
 set_ptracer_any
 sigaction