selftest: af_unix: Add test case for backtrack after finalising SCC.

syzkaller reported a KMSAN splat in __unix_walk_scc() while backtracking
edge_stack after finalising SCC.

Let's add a test case exercising the path.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: Shigeru Yoshida <syoshida@redhat.com>
Link: https://patch.msgid.link/20240702160428.10153-2-syoshida@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>